System and method for near-field electromagnetic ranging

ABSTRACT

A system for measuring distance between a first locus and a second locus includes: (a) at least one beacon device; a respective beacon device of the at least one beacon device being situated at the first locus and transmitting a respective electromagnetic signal; and (b) at least one locator device; a respective locator device of the at least one locator device being situated at the second locus and receiving the respective electromagnetic signal. The respective locator device is situated at a distance from the respective beacon device within near-field range of the respective electromagnetic signal. The respective locator device distinguishes at least two characteristics of the respective electromagnetic signal. The respective locator device employs the at least two characteristics to effect the measuring.

This application claims benefit of prior filed copending ProvisionalPatent Application Ser. No. 60/404,602, filed Aug. 19, 2002, andcopending Provisional Patent Application Ser. No. 60/404,604, filed Aug.19, 2002.

BACKGROUND OF THE INVENTION

The present invention relates generally to measurement of distance orranging by exploitation of near-field electromagnetic behavior andespecially to a system and method for evaluating a distance between atransmitter or beacon and a receiver or locator. Still morespecifically, the present invention describes a means for determining arange to a transmit-only beacon without requiring synchronization, andwithout relying on variation in signal amplitude. The present inventionmay be advantageously used as part of a more general system fordetermining position (range and bearing) or for tracking (determiningposition in near real time).

Related Art

A variety of techniques are known in the art for using electromagneticsignals to determine direction and distance. These techniques aresometimes referred to as radio direction finding and radio ranging. Agood summary of the state of the art in radio direction finding isprovided by Jenkins. [Small-Aperture Radio Direction-Finding, by HerndonH. Jenkins; Artech House, Boston; 1991; pp. 1-23.]

Time Difference and Phase Difference Angle of Arrival

One technique for radio direction finding has been dubbed timedifference of arrival (TDOA). This technique uses a pair of co-polarizedantennas separated by a baseline distance. An incoming signal incidentin a direction perpendicular to the baseline is received by bothantennas at the same time. When the direction of incidence is notperpendicular to the baseline, one antenna will receive the signalbefore the other. The difference in the time of arrival of the signalsat each antenna can be related to the angle of incidence. Equivalently,this difference in time of arrival may be treated in a manner similar tophase difference. Using this technique, the direction of arrival of anincident plane wave may be determined. This TDOA technique may begeneralized to apply to a network of receiving antennas at knownpositions. By comparing the times of arrival of the signal at eachreceiving antenna, the direction of the incident plane wave may bedetermined. In many (but not necessarily all) circumstances, thedirection from which the incoming plane wave arrives is the direction inwhich a target transmitter resides. Early examples of such radiodirection finding systems include the direction finding systemsdisclosed by J. S. Stone (U.S. Pat. No. 716,134; U.S. Pat. No. 716,135;U.S. Pat. No. 899,272; U.S. Pat. No. 961,265) and Roos (U.S. Pat. No.984,108). Phase detection for angle of arrival in the manner nowgenerally understood in the current art was disclosed by Fritz (U.S.Pat. No. 2,160,135) by Runge (U.S. Pat. No. 2,234,654) and by Budenbom(U.S. Pat. No. 2,423,437). 3-D radio direction finding using phasedifference was disclosed by Jansky (U.S. Pat. No. 2,437,695). Lioio etal (U.S. Pat. No. 5,724,047) disclose a phase and time difference radiodirection finding system.

Antenna Pattern Angle of Arrival

Another technique for radio direction finding involves using an antennawhose response varies as a function of angle. In one implementation, onemight use a directive antenna with a relatively narrow beam-width in aparticular boresight or direction of maximum signal strength. Theorientation of the antenna is varied until the received signal ismaximized so that the boresight of the antenna is aligned with thedirection of the incoming signal. In an alternate implementation, onemight use an antenna with a null in a particular null direction ordirection of minimum signal strength.

In one early invention, Erskine-Murray et al (U.S. Pat. No. 1,342,257)disclose the use of a loop antenna rotated about an axis lying in theplane of the loop. A similar apparatus that allowed finding a minimum ornull while still receiving a signal was disclosed by Robinson (U.S. Pat.No. 1,357,210). Two loop antennas with orthogonal axes may beelectrically combined so as to create a virtual antenna oriented in thedirection of a signal maximum (or minimum). A capacitive combiningarrangement or goniometer was disclosed by Bellini (U.S. Pat. No.1,297,313), and a transformer or inductive coupling goniometer wasdisclosed by Goldschmidt et al (U.S. Pat. No. 1,717,679). Anelectrically small loop and an electrically small whip (or dipole)antenna may be combined to yield a cardiod type pattern with a sharpnull in a particular azimuthal direction. The orientation of the antennamay be varied until the received signal is minimized, then the nulldirection is aligned with the direction of the incoming signal. Examplesof this technique are disclosed by Taylor (U.S. Pat. No. 1,991,473),Bailey (U.S. Pat. No. 1,839,290), and Busignies (U.S. Pat. No.1,741,282). The technique of goniometer combination of signals fromdirective antennas was also disclosed by Fischer (U.S. Pat. No.2,539,413).

Amplitude Comparison Angle of Arrival

Still another technique for determining the angle of arrival of a radiowave is amplitude comparison angle of arrival. The signal amplitudes oftwo or more antennas are compared so as to determine angle of arrival.For instance if a first antenna signal amplitude is very large and asecond antenna signal amplitude is small, one can infer that the radiowave arrived from the direction of the first antenna's pattern maximumand the second antenna's pattern minimum. If the signals are ofcomparable size, then the radio wave may have arrived from a directionin which the two antennas' patterns have comparable directivity. This issimilar to the traditional goniometer angle of arrival technique alreadymentioned. Examples of this technique include disclosures by Earp (U.S.Pat. No. 2,213,273), Wagstaffe (U.S. Pat. No. 2,213,874), Budenbom (U.S.Pat. No. 2,234,587), and Clark (U.S. Pat. No. 2,524,768).

Doppler Angle of Arrival

Yet another technique for radio direction finding takes advantage of theDoppler-Fizeau effect. If a receive antenna is rotated at high speedabout an axis perpendicular to the direction of an incoming signal, thenthat incoming signal will be shifted up in frequency as the receiveantenna moves toward the direction of the incoming signal and down infrequency as the receive antenna moves away from the direction of theincoming signal. In practice, it is not feasible to rotate an antenna ata high enough angular velocity for this effect to be readily observable.Instead, a number of receive antennas may be placed in a circle andsequentially scanned or sampled at a high rate in order to simulaterotation. Such systems were disclosed by Earp (U.S. Pat. No. 2,651,774)and Steiner (U.S. Pat. No. 3,025,522).

Hybrid Angle of Arrival

The prior art techniques discussed hereinabove for making angle ofarrival measurements may be advantageously combined. For example,Edwards et al (U.S. Pat. No. 2,419,946) disclose the combination ofamplitude and phase comparison in a radio direction finding system.Murphy et al (U.S. Pat. No. 5,541,608) disclose combining amplitude andphase comparison in a radio direction finding system. Murphy et al donot employ their disclosed architecture to measure range or distance,and they do not employ near-field behavior of electromagnetic signalingas is taught by the present invention.

Triangulation

A variety of radio direction finding measurements from a network of twoor more dispersed positions allows the location of a target transmitterto be determined. One technique by which this can be accomplishedemploys triangulation. For example, if the direction to a targettransmitter has been determined from three known positions, the bearingsfor the three directions may be plotted on a map, and the location ofthe target transmitter is at the intersection of the bearings, or by thetriangular region bounded by the intersections of the bearings. Anexample of such a system was disclosed by Maloney et al (U.S. Pat. No.4,728,959).

Radio Ranging

Radio ranging may be accomplished by triangulation from a collection ofdirection finding measurements. However, a disadvantage to this priorart ranging technique is that obtaining even a single range or distancecalculation requires measurements taken from at least two differentpositions. The positions must be separated by a baseline that is asignificant fraction of the range to be measured in order to obtain areliable range determination.

RADAR

There are a variety of other ways in which range may be measured. Onetechnique is RAdio Detection And Ranging (RADAR) such as is disclosed byPlaistowe (U.S. Pat. No. 2,207,267). The radar technique relies on thescattering of signals from a target. Radar works well in the detectionof aircraft in an open sky or ships on the surface of an ocean but radardetection becomes increasingly difficult when the target being trackedis in a cluttered environment populated by scatterers of equivalentcross-section to the target one desires to track.

Passive Tag Ranging

A passive cooperative target, passive transponder, or passive tag yieldsbetter performance than is achieved with an uncooperative radar target.In a passive tag ranging system, a transmitter radiates a signal that isreceived by a passive transponder. The passive transponder takes thereceived energy and reradiates the signal. The reradiated signal isreceived at the original transmitter and compared to the originaltransmitted signal. This comparison may involve phase, time delay, orother comparison between the transmitted and received signals whichenables a range measurement. An example of such a system is disclosed byLichtenberg et al (U.S. Pat. No. 4,757,315). A disadvantage of passivetag ranging is that the effective range tends to be relatively short dueto the low power picked up by the tag that is available to bereradiated.

Active Transponder Ranging

An active cooperative target is generally more effective in rangingoperations than a passive target. An active transponder listens for aparticular interrogatory signal and responds with a particular replysignal. The frequency of the reply signal is not necessarily the same asthe interrogatory signal, and the strength of the return signal is notdependent on the strength of the interrogatory signal received by thetarget. This technique may be referred to as active transponder ranging.The time of flight from an interrogating transmitter to the transponderand back to a receiver may be determined by a phase comparison of theoriginal transmitted signal with the signal received from the remotetransponder. In some embodiments, the phase comparison may be performedon a modulation imposed on an interrogatory signal and a reply signal.Knowing the wave velocity of signals, the time of flight may betranslated into a distance. Examples of transponder type ranging systemsinclude disclosures by Green (U.S. Pat. No. 1,750,668), Nicolson (U.S.Pat. No. 1,945,952), Gunn (U.S. Pat. No. 2,134,716), Holmes (U.S. Pat.No. 2,198,113), and Strobel (U.S. Pat. No. 2,248,727). Deloraine et al(U.S. Pat. No. 2,408,048) disclose a system for using time modulatedpulses in a transponder ranging system. Nosker (U.S. Pat. No. 2,470,787)discloses a system for 3-D position measurement using transponderranging, and Williams (U.S. Pat. No. 3,243,812) discloses a particularlysimple transponder system involving cycle counting of a phase comparisonbetween a transmitted signal and a received transponded signal. Adisadvantage of transponder ranging is that it requires an active targetto receive a signal, and transmission of a return signal is generallyinfluenced by some property of the received interrogatory signal.

Transmit-Only Ranging

A simpler transmit-only ranging scheme uses a transmit-only target. Oneway to implement a transmit-only ranging system is to measure theamplitude of signals received from a transmitter of known transmitpower. This amplitude ranging method of radio ranging was disclosed byde Forest (U.S. Pat. No. 749,436; U.S. Pat. No. 758,517; U.S. Pat. No.1,183,802). In some cases the amplitude decreases in a predictablefashion with distance from the receiver. For instance in free space,received power varies as the inverse square of the distance. Knowing thetransmit power, the receive power and the properties of the antennas,one can infer the range using a known relationship, such as Friis Law.

The relationship between transmitted power (P_(TX)) and received power(P_(RX)) in a far-field RF link is given by Friis Law: $\begin{matrix}{P_{RX} = {P_{TX}\quad\frac{G_{TX}G_{RX}\lambda^{2}}{4\quad\pi^{2}r^{2}}}} & \lbrack 1\rbrack\end{matrix}$

where G_(TX) is the transmit antenna gain,

-   -   G_(RX) is the receive antenna gain,    -   λ is the RF wavelength, and    -   r is the range between the transmitter and receiver.

Power rolls off (i.e., power decreases as range increases) in thefar-field as the inverse square of the distance$\left( \frac{1}{r^{2}} \right).$Near-field links do not obey this relationship. Near-field power rollsoff at powers higher than inverse square, typically inverse fourth$\left( \frac{1}{r^{4}} \right)$or higher.

This near-field behavior has several important consequences. First, theavailable power in a near-field link tends to be much higher than wouldbe predicted from the usual far-field, Friis Law relationship. Thisresults in a higher signal-to-noise ratio (SNR) and a better performinglink. Second, because the near-fields have such a relatively rapid powerroll-off, range tends to be relatively finite and limited. Thus, anear-field system is less likely to interfere with another RF systemoperating outside the operational range of the near-field system.

Inferring range from received signal power or amplitude is problematicat best. Despite the difficulties, amplitude ranging systems are stillused. For instance Moulin (U.S. Pat. No. 5,955,982) disclosed a methodand device for detecting and locating people buried under an avalanchein which signal amplitude is used to localize an avalanche victim.

There are a variety of other ways by which a receiver can obtain rangeinformation from a transmit-only target. Ranger (U.S. Pat. No.1,639,667) disclosed the idea of synchronized oscillators at atransmitter and at a remote receiver. A receiver can compare the numberof 360° phase shifts or the number of beats per time to infer a changein distance. In a series of inventions, Gage (U.S. Pat. No. 1,828,531;U.S. Pat. No. 1,939,685; U.S. Pat. No. 1,939,686; U.S. Pat. No.1,961,757) disclosed transmitting a pair of signals at differentfrequencies with different propagation characteristics and differentattenuation constants. By comparing the amplitude ratio of the receivedsignals, range may be inferred. Runge (U.S. Pat. No. 2,134,535)disclosed looking at the superposition of direct and reflected rays in areceived signal to infer range from a transmitter. Herson (U.S. Pat. No.2,314,883) disclosed evaluating the rate of change of the amplitude of areceived signal in order to infer range. Hammerquist (U.S. Pat. No.4,788,548) disclosed a multi-channel receiver for making phasemeasurements that allows a range measurement to be made. More recently,Sullivan (U.S. Pat. No. 5,999,131) disclosed a network of receiversisolating the direct path signal from a transmitter. Relative phasedifference measurements between receivers in the network are convertedinto differential range estimates for locating the transmitter.Sullivan's system has the disadvantage of requiring a common time baseor synchronization among all receivers in the network.

If a transmitter and a receiver are synchronized, then a precise phasemeasurement at a receiver can yield range information, up to a 360°phase uncertainty. In other words, a synchronized receiver can determinethe location of a transmitter relative to the start and end of awavelength, yet not be able to determine whether the transmitter'sposition lies within (for example) the seventh, or eighth or some otherwavelength away. If the transmitter's absolute (or reference) positionis determined initially by some other means, then the receiver can trackthe change in position of the transmitter relative to the establishedreference. Precise synchronization is essential to achieving meaningfulrange information in such a system. Any clock drift between thetransmit-only target and the receiver results in a range error. As apractical matter, however, precise synchronization is exceedinglydifficult and often expensive to achieve.

Transmit-only ranging may also be accomplished with an unsynchronizedtransmit-only target, using a network of synchronized receivers. Therelative difference in received phase can be translated into a relativedifference in position, subject to a 360° phase ambiguity.

All of these transmit-only ranging schemes rely on the “far-field”assumption: one must assume that a transmit-only target and a receiverare located at least a half wavelength apart. If a transmit-only targetand a receiver are located within a half wavelength or less of eachother, then near-field ambiguities make it difficult to determine anaccurate range.

The simplicity of a transmit-only ranging system is attractive. Howeverexisting transmit-only ranging systems suffer from significantdisadvantages. Some transmit-only ranging systems are dependent onprecise synchronization of a network of receivers that tend to becomplex, difficult, and expensive to implement. Some transmit-onlyranging systems are dependent on measurement of a precise time betweentransmission and reception in order to calculate distance by multiplyingtime and signal velocity. Some transmit-only ranging systems aredependent on a similarly difficult synchronization of transmitter andreceiver. Some transmit-only ranging systems are dependent oncalibrating a transmitter to a known position before an absolute rangecan be determined. Some transmit-only ranging systems are dependent on apredictable variation between range and amplitude seldom found in thereal world.

As far as the inventors are aware, prior art electromagnetic trackingand ranging systems are dependent on far-fields: radiatedelectromagnetic fields received at distances on the order of awavelength or (usually) much further. Even inventors such as Ranger(U.S. Pat. No. 1,639,667) who disclose operation at ranges on the orderof a wavelength or less implicitly assume far-field signal behavior. Noprior art ranging system known to the inventors exploits near-fieldsignal phenomena in performing ranging or distance measurement. Thepresent invention employs near-field signal phenomena to advantage andhas none of the dependencies and shortcomings noted in prior art rangingsystems.

Historical Context

Some of the earliest wireless communication systems involved near-fieldor inductive coupling. One example involved coupling telegraph signalsbetween a moving train and an adjacent telegraph line. With thediscoveries of Hertz (as put into practice by such innovators asMarconi, Lodge, and Tesla), the overwhelming emphasis of RF developmentfocused on long range, far-field systems. Frequencies were relativelylow by contemporary standards. The earliest development was in the lowfrequency (LF) band (30 kHz-300 kHz), and soon progressed to the mediumfrequency (MF) band (300 kHz-3 MHz), with some pioneering work extendinginto the high frequency (HF) band (3-30 MHz). This work was orientedtoward the empirical. Engineers focused on practical techniques forradiating and receiving signals. Little work was done to define orunderstand the fundamental physics that enables the radio frequency (RF)arts. For example, in 1932 the eminent RF expert, Frederick Terman,could say, “An understanding of the mechanism by which energy isradiated from a circuit and the derivation of equations for expressingthis radiation quantitatively involve conceptions which are unfamiliarto the ordinary engineer.” [Radio Engineering, First Edition, byFrederick Emmons Terman; McGraw Hill, Book Co., Inc., New York; 1932; p.494.] At that time the frontier of the RF arts had just begun to probethe lower end of the very high frequency (VHF) band (30 MHz-300 MHz).One textbook from the period provides a spectrum chart that ends with“30,000 kHz-60,000 kHz: Experimental and Amateur; >60,000 kHz: Not NowUseful”. [Radio Physics Course, Second Edition, by Alfred A. Ghirardi;Farrar & Rinehart, Inc., New York; 1942; p. 330.]

Radio direction finding and ranging remained focused on long range,far-field applications such as radio navigation for airplanes and radioguidance systems. The Japanese homed in on a Honolulu radio station intheir attack on Pearl Harbor [Joe Carr's Loop Antenna Handbook, FirstEdition, by Joseph J. Carr; Universal Radio Research, Reynoldsburg,Ohio; 1999; p. 85.] Only in the 1940's, with the development of RADAR,did a theoretical emphasis in the RF arts catch up with the longstandingempirical emphasis. By then however, the RF frontier had rapidly passedthrough VHF and UHF and moved on to microwaves. The LF, MF, and even theHF bands were increasingly a backwater far removed from the activeattention of most RF engineers.

In short, by the time fundamental electromagnetic theory began to beactively applied by RF engineers, RF engineers were not actively focusedon applying this theory to the problem of radio ranging at lowfrequencies such as those in the LF, MF, and HF band. By and large, theoverwhelming emphasis in the RF arts has been toward far-field systems,ones that operate at ranges beyond a wavelength, rather than near-fieldsystems that operate at ranges within a wavelength or so.

Lower frequencies have certain advantages over higher frequencies. Lowerfrequencies tend to diffract better around obstructions and thus can beused in non-line-of-sight applications such as over a hill or around abuilding. Because of the longer wavelengths associated with lowerfrequencies, multipath interference is far less of a problem than athigher frequencies. Further, lower frequencies tend to be morepenetrating of foliage and typical building materials, such as wood,brick, or concrete. Lower frequency RF circuits tend to be easier tobuild, and more robust. Components for use at lower RF frequencies tendto be less expensive and more readily available than those for use athigher frequencies.

Operation in the near-field, at ranges within a wavelength or so, yieldscertain advantages as well. Near-field signal levels tend to be farhigher than would be predicted from the usual inverse range square$\left( \frac{1}{r^{2}} \right)$far-field radiation relationships. In contrast, signal levels in thenear-field decrease more rapidly than in the far-field, decreasing inintensity as a function of $\frac{1}{r^{4}}.$As a result, there is a lesser problem with electromagnetic interferenceamong adjacent near-field systems so that it is easier to re-use thesame frequency in a smaller cell size than would be expected from theusual far-field predictions. In short, electromagnetic waves behavedifferently in the near-field than in the far-field, and the inventorshave discovered that the continuous and predictable variation of certainelectromagnetic parameters may be used as signals traverse thenear-field en route to the far-field to ascertain range or distanceinformation.

Despite these near-field advantages, to the best of the knowledge of theinventors, no prior art describes a system in which near-field signalphenomena and the predictable behavior of those phenomena as theytransition from near-field to far-field behavior are exploited in orderto obtain range or distance information.

There is a need for an electromagnetic ranging apparatus and method thatcan be operated asynchronously without requiring synchronization oftransmitter to receiver or synchronization among a network of receivers.

There is a further need for an electromagnetic ranging apparatus andmethod that can be operated without an awkward and lengthy calibrationprocess and that can be useful in a wide variety of propagationenvironments.

There is another need for an electromagnetic ranging apparatus andmethod that can be used as part of a location or position trackingsystem.

There is yet a further need for a system and method for finding therange to or position of a source of electromagnetic signals whoselocation is unknown.

There is still a further need for a system and method of electromagneticranging that operates using relatively low frequencies and takesadvantage of the characteristics of near-fields.

SUMMARY OF THE INVENTION

A system for measuring distance between a first locus and a second locusincludes: (a) at least one beacon device; a respective beacon device ofthe at least one beacon device being situated at the first locus andtransmitting a respective electromagnetic signal; and (b) at least onelocator device; a respective locator device of the at least one locatordevice being situated at the second locus and receiving the respectiveelectromagnetic signal. The respective locator device is situated at adistance from the respective beacon device within near-field range ofthe respective electromagnetic signal. The respective locator devicedistinguishes at least two characteristics of the respectiveelectromagnetic signal. The respective locator device employs the atleast two characteristics to effect the measuring.

A method for measuring distance between a first locus and a second locusincludes the steps of: (a) transmitting an electromagnetic signal fromthe first locus; (b) receiving the electromagnetic wave at the secondlocus; the second locus being within near-field range of theelectromagnetic signal; (c) in no particular order: (1) detecting afirst characteristic of the electromagnetic signal; and (2) detecting asecond characteristic of the electromagnetic signal; (d) measuring adifference between the first characteristic and the secondcharacteristic; and (e) employing the difference to calculate thedistance.

The electromagnetic ranging apparatus of the present invention employsnear-field electromagnetic behavior to measure a distance between atransmit beacon and a receive locator. A locator includes: (a) a firstreceiving antenna sensitive to electric (E) fields, (b) a secondreceiving antenna sensitive to magnetic (H) fields, (c) a means forreceiving a first signal from a first receiving antenna, (d) a means forreceiving a second signal from a second receiving antenna, (e) a meansfor determining a difference between a first signal and a second signal,and (f) a means for determining a distance of a beacon from a locatorusing a difference.

The present invention demonstrates that a phase difference betweenelectric and magnetic fields may be exploited to determine a range to abeacon, such as a transmitter or other source of electromagnetic wavesor signals. Typical implementations can determine a range to a beaconbetween about 0.05 λ and 0.50 λ away, where λ is the wavelength of theelectromagnetic signal transmitted by the beacon. Higher performanceimplementations of the present invention may operate at ranges less than0.05λ and greater than 0.50λ.

It is an object of the present invention to provide an electromagneticranging apparatus and method that can be operated asynchronously withoutrequiring synchronization of transmitter to receiver or synchronizationamong a network of receivers.

It is a further object of the present invention to provide anelectromagnetic ranging apparatus and method that can be operatedwithout an awkward and lengthy calibration process and that can beuseful in a wide variety of propagation environments.

Yet another object of the present invention is to provide anelectromagnetic ranging apparatus and method that can be used as part ofa location or position tracking system.

An additional object of the present invention is to provide a system andmethod for finding the range to or position of a source ofelectromagnetic waves whose location is unknown.

Still another object of the present invention is to provide a system andmethod of electromagnetic ranging that operates using relatively lowfrequencies and takes advantage of the characteristics of near-fields.

Further objects and features of the present invention will be apparentfrom the following specifications and claims when considered inconnection with the accompanying drawings, in which like elements arelabeled using like reference numerals in the various figures,illustrating the preferred embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of electric and magnetic field phaserelationships as a function of range for an ideal electrically smallloop in free space.

FIG. 2 is a table relating range of operation and frequency for anear-field ranging system.

FIG. 3 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase in quadrature.

FIG. 4 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase in phase synchrony.

FIG. 5 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase.

FIG. 6 is a schematic diagram of details of a preferred embodiment of asystem for near-field ranging by comparison of electric and magneticfield phase.

FIG. 7 is a schematic diagram of a system for near-field ranging bycomparison of electric and magnetic field phase with beacon and locatorfunction combined in a single unitary device.

FIG. 8 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization omni-directionallocator.

FIG. 9 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization omni-directionallocator.

FIG. 10 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization directional locator.

FIG. 11 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization directional locator.

FIG. 12 is a schematic diagram illustrating details of an exemplaryreceiver in a system for electromagnetic ranging.

FIG. 13 is a schematic diagram illustrating a near-field ranging systemconfigured according to a fixed beacon-mobile locator architecture.

FIG. 14 is a schematic diagram illustrating a near-field ranging systemconfigured according to a fixed/mobile locator-mobile beaconarchitecture.

FIG. 15 is a schematic diagram illustrating a near-field ranging systemconfigured according to a reciprocal beacon-locator architecture.

FIG. 16 is a schematic diagram illustrating a near-field ranging systemconfigured employing a passive tag architecture.

FIG. 17 is a schematic diagram illustrating a near-field ranging systemconfigured employing a near-field remote sensing architecture.

FIG. 18 is a flow diagram illustrating the method of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview of the Invention

The present invention will now be described more fully in detail withreference to the accompanying drawings, in which the preferredembodiments of the invention are shown. This invention should not,however, be construed as limited to the embodiments set forth herein;rather, they are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of the invention to thoseskilled in art. Like numbers refer to like elements throughout.

An Analytic Model

Suppose a transmit-only target uses a small loop antenna that behaveslike a time domain magnetic dipole. A magnetic dipole may be thought ofas a small current loop of area A, and a time dependent current I=I₀T(t) where I₀ is an initial or characteristic current and T(t) is thetime dependence. Assume the dipole lies in the x-y plane centered at theorigin with its axis in the z direction. The dipole's magnetic moment mis: m=A I₀ T(t), or m=m₀ T(t). The magnetic field or “H-field” of thissmall loop is: $\begin{matrix}{{{H(t)} = {{\frac{m_{0}}{4\quad\pi\quad r^{2}}\left( {\frac{T}{r} + \frac{\overset{.}{T}}{c}} \right)\left( {{2\quad\cos\quad\theta\quad\hat{r}} + {\sin\quad\theta\quad\hat{\theta}}} \right)} + {\frac{m_{0}\overset{¨}{T}\quad\sin\quad\theta}{4\quad\pi\quad c^{2}r}\hat{\theta}}}},} & \lbrack 2\rbrack \\{{and}\quad{the}\quad{electric}\quad{field}\quad{or}\quad{``{E\text{-}{field}}"}\quad{is}\text{:}} & \quad \\{{{E(t)} = {{- \frac{1}{4\quad\pi\quad ɛ_{0}}}\frac{m}{c^{2}r}\left( {\frac{\overset{.}{T}}{r} + \frac{\overset{¨}{T}}{c}} \right)\sin\quad\theta\quad\hat{\varphi}}},} & \lbrack 3\rbrack\end{matrix}$where r is the range from the origin, c is the speed of light, ε₀ is thepermeability of free space, and derivatives with respect to time aredenoted by dots. Assume a sinusoidal excitation T(t)=sin ωt where ω isthe angular frequency: ω=2πf. Then, {dot over (T)}(t)=ω cos ωt, {umlautover (T)}(t)=−ω² sin ωt, $\begin{matrix}{{{H(t)} = {{\frac{m_{0}}{4\quad\pi\quad r^{2}}\left( {\frac{\sin\quad\omega\quad t}{r} + \frac{\omega\quad\cos\quad\omega\quad t}{c}} \right)\left( {{2\quad\cos\quad\theta\quad\hat{r}} + {\sin\quad\theta\quad\hat{\theta}}} \right)} - {\frac{m_{0}\omega^{2}\quad\sin\quad\omega\quad t\quad\sin\quad\theta}{4\quad\pi\quad c^{2}r}\hat{\theta}}}},} & \lbrack 4\rbrack \\{{and}\text{:}} & \quad \\{{E(t)} = {{- \frac{1}{4\quad\pi\quad ɛ_{0}}}\frac{m_{0}}{c^{2}r}\left( {{\frac{\omega}{r}\quad\cos\quad\omega\quad t} - {\frac{\omega^{2}}{c}\quad\sin\quad\omega\quad t}} \right)\sin\quad\theta\quad{\hat{\varphi}.}}} & \lbrack 5\rbrack\end{matrix}$

There are a variety of ways in which range information may be obtainedfrom near-fields. For instance, one could compare a longitudinal orradial ({circumflex over (r)}) component of a first field to atransverse component ({circumflex over (θ)} or {circumflex over (φ)}) ofa first field. One could compare a longitudinal or radial ({circumflexover (r)}) component of a first field to a transverse component({circumflex over (θ)} or {circumflex over (φ)}) of a second field. Onecould compare a longitudinal or radial ({circumflex over (r)}) componentof a first field to a longitudinal or radial ({circumflex over (r)})component of a first field. One could compare a longitudinal or radial({circumflex over (r)}) component of a first field to a longitudinal orradial ({circumflex over (r)}) component of a second field. One couldcompare a transverse component ({circumflex over (θ)} or {circumflexover (φ)}) of a first field to a transverse component ({circumflex over(θ)} or {circumflex over (φ)}) of a first field. One could compare atransverse component ({circumflex over (θ)} or {circumflex over (φ)}) ofa first field to a transverse component ({circumflex over (θ)} or{circumflex over (φ)}) of a second field. These comparisons may includecomparisons of phase, comparisons of amplitude, or comparisons of othersignal properties.

The inventors have discovered that one particularly advantageous anduseful comparison is a comparison of phase of an electric component ofan electromagnetic wave to phase of a magnetic component of anelectromagnetic wave.

For this ideal small loop in free space, E-field phase in degrees as afunction of range is: $\begin{matrix}{\phi_{E} = {\frac{180}{\pi}{\left( {\frac{\omega\quad r}{c} + {\cot^{- 1}\quad\frac{\omega\quad r}{c}}} \right).}}} & \lbrack 6\rbrack\end{matrix}$

Transverse H-field phase in degrees as a function of range is:$\begin{matrix}{\phi_{H} = {\frac{180}{\pi}{\left( {\frac{\omega\quad r}{c} + {\cot^{- 1}\left( {\frac{\omega\quad r}{c} - \frac{c}{\omega\quad r}} \right)}} \right).}}} & \lbrack 7\rbrack\end{matrix}$

Note that Equation (6) has a branch cut at a range$r = {\frac{1}{2\quad\pi}\quad{\lambda.}}$The phase delta is given by: $\begin{matrix}{\Delta_{\phi} = {{\phi_{H} - \phi_{E}} = {\frac{180}{\pi}{\left( {{\cot^{- 1}\left( {\frac{\omega\quad r}{c} - \frac{c}{\omega\quad r}} \right)} - {\cot^{- 1}\quad\frac{\omega\quad r}{c}}} \right).}}}} & \lbrack 8\rbrack\end{matrix}$

These relations assume a measurement in the plane of the loop (θ=90°).Similar relations may be derived for other orientations.

FIG. 1 is a graphic representation of electric and magnetic field phaserelationships as a function of range for an ideal electrically smallloop in free space. In FIG. 1, a graphic plot 100 includes a magnetic orH-Field phase curve 102, an electric or E-Field phase curve 104 and aphase difference or Δφ curve 106 representing the difference betweencurves 102, 104. Curves 102, 104, 106 are plotted against a first axis108 representing phase (preferably in degrees) as a function of rangerepresented on a second axis 110 in wavelength (preferably in akilogram-meter-second unit, such as meters) of an electromagnetic signalunder consideration. Thus, the relations of Equations [6]-[8] areillustrated in graphical representation 100. H-field phase curve 102,described by Equation [7], begins 90° out of phase with respect toE-field phase 104, described by Equation [6]. As range is increased fromabout 0.05λ to about 0.50λ, H-field phase curve 102 initially decreases,then increases. Similarly, as range is increased from about 0.05λ toabout 0.50λ, E-field phase curve 104 increases, gradually at first, andat an increasing rate as range increases. The difference between E-fieldphase curve 104 and H-field phase curve 102 is represented by Δφ curve106. Δφ curve 106 begins at approximately 90° (i.e., at phasequadrature) in the near-field within a range of about 0.05λ and goes to0° (i.e., phase synchronicity) as the far-field is approached, past arange of about 0.50λ. Δφ curve 106 is described mathematically inEquation [8]. Transition of Δφ curve 106 from phase quadrature to phasesynchronicity between about 0.05λ to about 0.50λ is substantiallycontinuous and predictable and is used to advantage by the presentinvention. With more precise measurement, this phase transition can bebeneficially used at ranges inside 0.05λ and outside 0.50λ.

Equation [8] expresses phase difference Δφ as a function of range (r).Equation [8] is a transcendental relation that may not be inverted toyield an expression for range as a function of phase difference.Nevertheless, a variety of mathematical methods may be used to determinea range given a phase difference. Equation [8] may be advantageouslyemployed by other mathematical techniques such as, by way of example andnot by way of limitation, solving numerically, generating a look-uptable, and solving graphically.

In the far-field, at distances greater than one wavelength, both theelectric and magnetic fields are phase synchronous. The phase of eachfield varies in lock step with the other field at a fixed rate of 360°per wavelength in the far-field limit. This is the usual relationshipexpected by those skilled in the RF arts. As a rule, the near-fieldphase anomalies exploited by the preferred embodiment of presentinvention are rarely discussed, if at all, in the prior art. Oneexception to this rule is the work of one of the inventors.[Electromagnetic Energy Around Hertzian Dipoles, by H. Schantz; IEEEAntennas and Propagation Magazine, April 2001; pp. 50-62.]

FIG. 2 is a table relating range of operation and frequency for anear-field ranging system. In FIG. 2, a table 200 relates frequency withselected ranges expressed in terms of wavelength of a signal underconsideration. An important feature of the present invention is that aphase difference Δφ between electric and magnetic fields may beexploited to determine a range from a locator receiver to a beacontransmitter, or other source of electromagnetic waves. This near-fieldranging method allows a distance to a beacon to be accurately determinedbetween about 0.05 λ and 0.50 λ from the beacon, where λ is thewavelength of electromagnetic signal transmitted by a beacon. Optimumperformance is obtained from a range of about 0.08 λ to a range of about0.30 λ from the beacon. With more precise measurement, this phasetransition can be used for ranges inside 0.05λ and outside 0.50λ. Acorresponding characteristic range of operation as a function offrequency is presented in table 200; FIG. 2. Lower frequencies permitoperation at longer ranges; higher frequencies are preferred for shorterranges. The particular frequencies listed in table 200 (FIG. 2) arepresented for purposes of illustration and not for purposes oflimitation.

Determination of a range from a phase difference αφ between an electricand a magnetic field may be more complicated than the free space resultof Equation [8] indicates. In practice, one may wish to calibrate aranging system using a more complicated analytical or computationalmodel (for example, a model including the effect of propagation over areal ground instead of free space), or using experimental data from anenvironment within which one wishes to carry out ranging operations.

The present invention allows ranging to at least 3000 feet in the160-190 kHz band, to at least 900 feet in the AM radio band, and toshorter ranges at higher frequencies. A wide variety of otheroperational ranges are available by using other frequencies. Greaterrange can be achieved with lower frequency. Accuracy within inches isachievable even at the longest ranges.

In the interest of presenting a simple illustrative example of thepresent invention, that is by way of illustration and not by way oflimitation, this description addresses a mobile beacon and a stationarylocator, but one skilled in the art may easily recognize that a beaconmay be fixed and the locator mobile, or both beacon and locator may bemobile. To avoid unnecessary prolixity in the discussion that follows,sometimes only a single locator and a single beacon are discussed. Thisshould not be interpreted so as to preclude a plurality of beacons andlocators used as part of a more complicated positioning, locating, ortracking system.

A System For Near-Field Ranging

FIG. 3 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase in quadrature. In FIG.3, a ranging system 300 is illustrated for near-field ranging bycomparison of electric and magnetic field phase with the electric andmagnetic field signals in quadrature (90° out of phase) at close range.A beacon 310 includes a transmitter 312 and a transmit antenna 337.Beacon 310 transmits an electromagnetic wave or signal 315 having awavelength λ.

A locator 320 receives electromagnetic signal 315. Locator 320 includesa first electric field antenna 332 for receiving an E-field signal 301and a second magnetic field antenna 331 which receives an H-field signal302. If a distance 304 between beacon 310 and locator 320 is, forexample, 0.05λ, then E-field signal 301 and H-field signal 302 areapproximately 90° out of phase at antennas 331, 322. Locator 320measures this phase difference Δφ and indicates that distance equals0.05λ in a distance indicator 306.

FIG. 4 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase in phase synchrony. InFIG. 4, a ranging system 400 is illustrated for near-field ranging bycomparison of electric and magnetic field phase with the electric andmagnetic field signals in phase synchronicity (0° phase difference) atfar range. A beacon 410 includes a transmitter 412 and a transmitantenna 437. Beacon 410 transmits an electromagnetic signal 415 having awavelength λ.

A locator 420 receives electromagnetic signal 415. Locator 420 has afirst electric field antenna 432 which receives an E-field signal 401,and a second magnetic field antenna 431 which receives an H-field signal402. If distance 404 between beacon 410 and locator 420 is 0.50λ, thenE-field signal 401 and H-field signal 402 are approximately 0° out ofphase (in phase synchronicity). Locator 420 measures this phasedifference αφ and indicates that distance equals 0.05λ in a distanceindicator 406.

Either locator 320, 420 may use the free space relationship betweenphase difference Δφ and range r described mathematically in Equation[8], may use a more exact analytic expression taking into account theeffects of soil and ground propagation, may use a theoretical simulationof the propagation environment, or may use empirical data regardingphase difference and range in a particular propagation environment oranother basis for determining the relationship between phase differenceΔφ and range r.

Basic Architecture of a System for Near-Field Ranging

FIG. 5 is a schematic illustration of a system for near-field ranging bycomparison of electric and magnetic field phase. In FIG. 5, a rangingsystem 500 is illustrated for near-field ranging by comparison ofelectric and magnetic field phase with the electric and magnetic fieldsignals. A beacon 510 includes a transmitter 512 and a transmit antenna536. Beacon 510 may be mobile, or fixed, or even an unknown oruncooperative source of electromagnetic radiation in the form of anelectromagnetic signal 515. Transmit antenna 536 can be a loopstickantenna or another type antenna that is substantially unaffected bychanges in an adjacent propagation environment. Transmit antenna 536could also be a whip antenna that is as large as is allowed by eitherpertinent regulations or the constraints imposed by a particularapplication. Beacon 510 transmits electromagnetic signal 515.

A locator 520 is situated a distance r from beacon 510 and receiveselectromagnetic signal 515. Locator 520 includes a first antenna 531, afirst receiver 525, a second antenna 532, a second receiver 527, asignal comparator 580, and a range detector 590. Signal comparator 580receives a first representative signal from first receiver 525 and asecond representative signal from second receiver 527. Signal comparator580 receives the first and second representative signals and identifiesa difference between the first and second representative signals. Theidentified difference may be a difference in phase, a difference inamplitude, or another difference between the first and secondrepresentative signals. Signal comparator 580 generates a third signalproportional to or otherwise related to the difference identified bysignal comparator 580. Range detector 590 receives the third signal fromsignal comparator 580 and employs the received third signal to determinerange r between beacon 510 and locator 520.

In the preferred embodiment of the present invention, first antenna 531is configured to permit first receiver 525 to generate the firstrepresentative signal provided to signal comparator 580 as a signalproportional to or otherwise representative of a first component ofelectromagnetic signal 515. Further in the preferred embodiment of thepresent invention, second antenna 532 is configured to permit secondreceiver 527 to generate the second representative signal provided tosignal comparator 580 as a signal proportional to or otherwiserepresentative of a second component of electromagnetic signal 515. Thefirst component and second component of electromagnetic signal 515 maydiffer in polarization or some other detectable property. One differenceadvantageous in a near-field ranging system is a difference between alongitudinal or radial ({circumflex over (r)}) component and atransverse component ({circumflex over (θ)} or {circumflex over (φ)}) ofelectromagnetic signals.

In another preferred embodiment of the present invention, first antenna531 is an electric or E-field antenna that permits first receiver 525 togenerate the first representative signal provided to signal comparator580 as a signal proportional to or otherwise representative of a firstcomponent of electromagnetic signal 515, and second antenna 532 is amagnetic or H-field antenna that permits second receiver 527 to generatethe second representative signal provided to signal comparator 580 as asignal proportional to or otherwise representative of a second componentof electromagnetic signal 515.

In the most preferred embodiment of the present invention, first antenna531 is an H-field antenna, first receiver 525 is an H-field receiver,second antenna 532 is an E-field antenna, second receiver 527 is anE-field receiver, signal comparator 580 is a phase detector and rangedetector 590 employs phase information received from signalcomparator-phase detector 580 to determine range r between beacon 510and locator 520.

Thus, in the most preferred embodiment of the present invention first(H-field) antenna 531 responsive to a magnetic or H-field component ofelectromagnetic signal 515 and permits first (H-field) receiver 525 todetect a first signal proportional to the magnetic or H-field componentof electromagnetic signal 515. Antennas responsive to a magnetic orH-field component of an electromagnetic signal include, by way ofexample and not by way of limitation, loop and loopstick antennas. First(H-field) receiver 525 receives a signal from first (H-field) antenna531 and generates a first representative signal proportional to themagnetic or H-field component of electromagnetic signal 515. Therepresentative signal may, for example, be an analog signal having avoltage that is directly proportional to amplitude of the magnetic orH-field component of electromagnetic signal 515. Alternatively, therepresentative signal may be, for example, a digital signal conveyingdata pertaining to the magnetic or H-field component of electromagneticsignal 515. First (H-field) receiver 525 may include filtering,amplification, analog to digital conversion, and tuning means of thekind that are generally understood by practitioners of the RF arts.

Second (E-field) antenna 532 responsive to an electric or E-fieldcomponent of electromagnetic signal 515 allows second (E-field) receiver527 to detect a second signal proportional to an electric or E-fieldcomponent of electromagnetic signal 515. Antennas responsive to anelectric or E-field component of an electromagnetic wave include, by wayof example and not by way of limitation, whip, dipole, or monopoleantennas. Second (E-field) receiver 527 detects an input signal fromsecond (E-field) antenna 532 and yields a second signal proportional tothe electric or E-field component of electromagnetic signal 515. Therepresentative signal may, for example, be an analog signal whosevoltage is directly proportional to amplitude of the electric or E-fieldcomponent of electromagnetic signal 515. Alternatively, therepresentative signal may be, for example, a digital signal conveyingdata pertaining to the electric or E-field component of electromagneticsignal 515. Second (E-field) receiver 527 may include filtering,amplification, analog to digital conversion, and tuning means of thekind that are generally understood by practitioners of the RF arts.

If electromagnetic signal 515 is a single frequency sine wave, it isdesirable for a first (H-field) receiver 525 and a second (E-field)receiver 527 to employ a very narrow bandwidth filter so as to minimizethe noise and maximize the signal to noise ratio. However, it is alsoimportant for filters used in a first (H-field) receiver 525 and asecond (E-field) receiver 527 to have a constant passband group delay sothat relative phase characteristics of a first representative signal anda second representative signal are stable and predictable. The inventorshave advantageously employed Bessel filters as a starting point foroptimization.

First (H-field) antenna 531 and second (E-field) antenna 532 arepreferably oriented to be maximally responsive to polarization ofelectromagnetic signal 515. In alternate embodiments, locator 520 mayemploy additional (H-field) antennas, additional (E-field) antennas,additional H-field receivers, and additional E-field receivers in orderto detect multiple polarizations or so as to detect electromagneticsignals from additional incident directions. Because electromagneticsignal 515 has near-field characteristics, polarizations mayadvantageously include a longitudinal polarization with a componentparallel to a direction of travel of an incident electromagnetic signal.

Signal comparator 580 (preferably embodied in a phase detector) takesthe first representative signal proportional to the magnetic or H-fieldcomponent of electromagnetic signal 515 and the second representativesignal proportional to the electric or E-field component ofelectromagnetic signal 515 and determines a phase difference between thefirst and second representative signals. Phase detector 580 may bethought of (for purposes of illustration and not limitation) as a mixerthat receives the first and second representative signals and produces aquasi-static signal proportional to a quasi-static phase differencebetween the first and second representative signals. In an alternateembodiment, phase detector 580 may be implemented with an AND gatehaving as inputs the first and second representative signals and whoseoutput is provided to an integrator. The output of the integrator is aquasi-static signal proportional to a quasi-static phase differencebetween the first representative signal and the second representativesignal. The term “quasi-static” in this context means varying on a timescale substantially similar to a variation in phase, not necessarily atime scale or period substantially similar to that of electromagneticsignal 515. In other embodiments, phase detector 580 may receive orcapture a time domain signal and detect zero crossings or othercharacteristics of wave shape in order to determine an effective phasedifference between the first representative signal and the secondrepresentative second signal. Suitable phase detectors are readilyavailable—such as, by way of example and not by way of limitation, anAnalog Devices part no. AD 8302. Another embodiment of phase detector580 may take digital information from first (H-field) receiver 525 andsecond (E-field) receiver 527 and calculate a phase difference betweenthe first digital information and the second digital information.

Range detector 590 may be embodied in any means capable of converting ameasured phase difference to a range r. In a particular simple example,range detector 590 may be an analog voltmeter having a scale calibratedto read a range r as a function of an applied voltage from phasedetector 580. A more sophisticated embodiment of range detector 590 may,for example, advantageously employ an analog to digital converter and amicro-controller or micro-processor to calculate a range r from anapplied voltage received from phase detector 580. Range detector 590 mayinclude visual, audio, or other outputs to indicate range r to a user,or may convey a measured range r to a remote location for furtheranalysis as part of a comprehensive tracking, positioning, or locatingsystem.

Locator 520 may be generally regarded as comprising a means fordetecting and receiving a first signal, a means for detecting andreceiving a second signal, a means for determining a difference betweena first and a second representative signal related to the first andsecond signals and a means for determining a range given a differencebetween the first and second representative signals.

Beacon 510 may be generally regarded as comprising a means fortransmitting an electromagnetic signal. Beacon 510 may be a fixedreference with respect to which a mobile locator 520 determines adistance or range r. Alternatively, a fixed locator 520 may measurerange r of a mobile beacon 510, or a locator 520 may be a mobile unitthat measures range r of a mobile beacon 510. Furthermore, beacon 510may be an uncooperative transmitter or other source of anelectromagnetic signal 515 whose range r one desires to know withrespect to the position of a locator 520.

A Preferred Embodiment

FIG. 6 is a schematic diagram of details of a preferred embodiment of asystem for near-field ranging by comparison of electric and magneticfield phase. In FIG. 6, a ranging system 600 includes a beacon 610 and alocator 620 separated from beacon 610 by a range r. Beacon 610 includesa transmitter 612 which may be mobile or fixed, and a transmit antenna636. Transmitter 612 may include means to change properties of atransmitted electromagnetic signal 615 including, by way of illustrationand not by way of limitation, changing frequency, phase, polarization,or amplitude of an electromagnetic signal 615 according to apredetermined pattern, in response to an input or stimulus, such as, forexample, a control signal received from a data bus 695. In alternateembodiments, transmitter 612 may modulate a transmitted electromagneticsignal 615 so as to convey information. Such information may includeinformation that identifies beacon 610 or other information or telemetryof value to a user. For example, binary phase shift keying may beimplemented on a transmitted electromagnetic signal 615 withoutimpairing ranging performance of the present invention. In still anotherembodiment, transmitter 612 may turn on or off according to apredetermined pattern, in response to a control signal from a data bus695, or in response to some other input or stimulus. Such input orstimulus may include (but is not necessarily limited to) a signal froman accelerometer, a timer, a motion detector, other transducers or othersensors.

It may be advantageous in some applications for transmitter 612 tooperate at a higher instantaneous power and a lower duty cycle. Forinstance, transmitter 612 might operate at approximately ten times anallowed average power level but only transmit 10% of a characteristicperiod, thus maintaining a substantially similar average transmit powerlevel. Such intermittent operation would enable a higher signal to noiseratio (SNR) signal. Periodic operation of beacon 610 is alsoadvantageous for operation in the presence of interference. When beacon610 is silent (i.e., not transmitting), locator 620 can characterize aparticular coherent noise source such as an interfering signal and cancompensate for the presence of this coherent noise once beacon 610begins transmitting again.

In applications where security is particularly important, beacon 610 mayemploy techniques to make electromagnetic signal 615 more difficult todetect by an eavesdropper. These techniques may include a frequencyhopping scheme, periodic operation, varying transmit power to use theminimum power needed to make an accurate measurement, or other means torender signal 615 less detectable by an eavesdropper. Transmit powercontrol may be further advantageous to allow frequency reuse in smallercell sizes.

A first step in determining range r between beacon 610 and locator 620is for a beacon 610 to transmit an electromagnetic signal 615. In apreferred embodiment, electromagnetic signal 615 is verticallypolarized, but horizontal polarization or alternate polarizations areusable as well. To avoid unnecessary complication the electromagneticcoupling between beacon 610 and locator 620 is described in terms of anelectromagnetic wave comprising electromagnetic signal 615. Becauserange r between beacon 610 and locator 620 is typically less than awavelength of electromagnetic signal 615, electromagnetic signal 615 isnot typically a radiation electromagnetic wave decoupled from beacon 610such as would be found in the far-field at a range r significantlygreater than one wavelength of electromagnetic signal 615. It should beunderstood that an electromagnetic wave comprising electromagneticsignal 615 is typically a reactive or coupled electromagnetic wave,rather than a radiation or decoupled electromagnetic wave.

Locator 620 receives electromagnetic signal 615. In a preferredembodiment, locator 620 includes a first (H-field) channel 625, a second(H-field) channel 626, a third (E-field) channel 627, a local oscillator650, a first phase detector 681, a second phase detector 682, and arange detector 690 (including an analog to digital (A/D) converter 691,and a microprocessor 692). An optional data bus 695 may be used toprovide a means for exchanging control and data signals among aplurality of beacons and locators (not shown in detail in FIG. 6).

First (H-field) channel 625 includes a first (H-field) antenna 630, afirst (H-field) pre-select filter 6400, a first (H-field) mixer 6420, afirst (H-field) primary IF filter 6430, a first (H-field) primary IFamplifier 6440, a first (H-field) secondary IF filter 6450, a first(H-field) secondary IF amplifier 6460, and a first (H-field) automaticgain control 6480. First (H-field) channel 625 has a first (H-field)antenna port 6270, a first (H-field) tuning port 6230, a first (H-field)received signal strength indicator (RSSI) port 6220, and a first(H-field) signal output port 6210.

A second (H-field) channel 626 includes a second (H-field) antenna 631,a second (H-field) pre-select filter 6401, a second (H-field) mixer6421, a second (H-field) primary IF filter 6431, a second (H-field)primary IF amplifier 6441, a second (H-field) secondary IF filter 6451,a second (H-field) secondary IF amplifier 6461, and a second (H-field)automatic gain control 6481. Second (H-field) channel 626 has a second(H-field) antenna port 6271, a second (H-field) tuning port 6231, asecond (H-field) received signal strength indicator (RSSI) port 6221,and a second (H-field) signal output port 6211.

A third (E-field) channel 627 includes a third (E-field) antenna 632, athird (E-field) pre-select filter 6402, a third (E-field) mixer 6422, athird (E-field) primary IF filter 6432, a third (E-field) primary IFamplifier 6442, a third (E-field) secondary IF filter 6452, a third(E-field) secondary IF amplifier 6462, and a third (E-field) automaticgain control 6482. Third (E-field) channel 627 has a third (E-field)antenna port 6272, a third (E-field) tuning port 6232, a third (E-field)received signal strength indicator (RSSI) port 6222, and a third(E-field) signal output port 6212.

First (H-field) antenna 630 is responsive to the magnetic or H-fieldcomponent of electromagnetic signal 615 and presents a received signalproportional to the magnetic or H-field component of electromagneticsignal 615 to first (H-field) pre-select filter 6400.

First (H-field) pre-select filter 6400 passes a first representativesignal proportional to the magnetic or H-field component ofelectromagnetic signal 615, but rejects signals with undesirablefrequencies. First (H-field) pre-select filter 6400 may be, for example,a band pass filter or a low pass filter. Typically first (H-field)pre-select filter 6400 will pass those frequencies within which beacon610 might transmit an electromagnetic signal 615 for a relevantapplication. Selection of a band will depend upon a variety of factorsincluding, but not necessarily limited to, regulatory constraints,propagation behavior of electromagnetic signal 615, and a desired ranger of operation.

First (H-field) mixer 6420 mixes the first representative signalreceived from first (H-field) pre-select filter 6400 with a localoscillator (LO) signal generated by local oscillator 650 to generate afirst intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

First primary (H-field) IF filter 6430 accepts only the desired first IFrepresentative signal and rejects other undesired signals. A crystalfilter may be advantageously used as first primary (H-field) IF filter6430. Such a crystal filter is characterized by an extremely narrow passband, and preferably has a constant group delay within the pass band. Anarrow pass band acts to allow the desired first IF representativesignal to be conveyed to first primary (H-field) IF amplifier 6440 whilerejecting adjacent undesired signals. First primary (H-field) IFamplifier 6440 increases the amplitude of the first IF representativesignal and conveys the amplified first IF representative signal to firstsecondary (H-field) IF filter 6450. First secondary (H-field) IF filter6450 accepts only the desired first IF representative signal and rejectsother undesired signals. A crystal filter may be advantageously used asfirst secondary (H-field) IF filter 6450. Such a crystal filter ischaracterized by an extremely narrow pass band, and preferably has aconstant group delay within the pass band. A narrow pass band acts so asto allow the desired first IF representative signal to be conveyed tofirst secondary (H-field) IF amplifier 6460 while rejecting adjacentundesired signals. First secondary (H-field) IF amplifier 6460 increasesthe amplitude of the first IF representative signal and conveys thefirst IF representative signal to signal output port 6210 and to firstautomatic gain control (AGC) 6480.

First automatic gain control 6480 adjusts a gain of first primary(H-field) IF amplifier 6440 and first secondary (H-field) IF amplifier6460 to maintain a desired level of the first IF representative signal.By dividing a desired total gain between first primary (H-field) IFamplifier 6440 and first secondary (H-field) IF amplifier 6460, a hightotal gain and a large dynamic range can be maintained with greaterstability and reliability than in a single amplification stage alone.Similarly, by dividing the desired filtering between first primary(H-field) IF filter 6430 and first secondary (H-field) IF filter 6450, amore narrow passband can be achieved with greater stability and greaterreliability than with a single filter stage alone. First automatic gaincontrol 6480 preferably includes a received signal strength indicator(RSSI) and conveys an RSSI level to RSSI output 6220.

Second (H-field) antenna 631 is responsive to the magnetic or H-fieldcomponent of electromagnetic signal 615 and presents a received signalproportional to the magnetic or H-field component of electromagneticsignal 615 to second (H-field) pre-select filter 6401.

Second (H-field) pre-select filter 6401 passes a first representativesignal proportional to the magnetic or H-field component ofelectromagnetic signal 615, but rejects signals with undesirablefrequencies. Second (H-field) pre-select filter 6401 may be, forexample, a band pass filter or a low pass filter. Typically second(H-field) pre-select filter 6401 will pass those frequencies withinwhich beacon 610 might transmit an electromagnetic signal 615 for arelevant application. Selection of a band will depend upon a variety offactors including, but not necessarily limited to, regulatoryconstraints, propagation behavior of electromagnetic signal 615, and adesired range r of operation.

Second (H-field) mixer 6421 mixes the first representative signalreceived from second (H-field) pre-select filter 6401 with a localoscillator (LO) signal generated by local oscillator 650 to generate asecond intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

Second primary (H-field) IF filter 6431 accepts only the desired secondIF representative signal and rejects other undesired signals. A crystalfilter may be advantageously used as second primary (H-field) IF filter6431. Such a crystal filter is characterized by an extremely narrow passband, and preferably has a constant group delay within the pass band. Anarrow pass band acts to allow the desired second IF representativesignal to be conveyed to second primary (H-field) IF amplifier 6441while rejecting adjacent undesired signals. Second primary (H-field) IFamplifier 6441 increases the amplitude of the second IF representativesignal and conveys the amplified second IF representative signal tosecond secondary (H-field) IF filter 6451. Second secondary (H-field) IFfilter 6451 accepts only the desired second IF representative signal andrejects other undesired signals. A crystal filter may be advantageouslyused as second secondary (H-field) IF filter 6451. Such a crystal filteris characterized by an extremely narrow pass band, and preferably has aconstant group delay within the pass band. A narrow pass band acts so asto allow the desired second IF representative signal to be conveyed tosecond secondary (H-field) IF amplifier 6461 while rejecting adjacentundesired signals. Second secondary (H-field) IF amplifier 6461increases the amplitude of the second IF representative signal andconveys the second IF representative signal to signal output port 6211and to second automatic gain control (AGC) 6481.

Second automatic gain control 6481 adjusts a gain of second primary(H-field) IF amplifier 6441 and second secondary (H-field) IF amplifier6461 to maintain a desired level of the second IF representative signal.By dividing a desired total gain between second primary (H-field) IFamplifier 6441 and second secondary (H-field) IF amplifier 6461, a hightotal gain and a large dynamic range can be maintained with greaterstability and reliability than in a single amplification stage alone.Similarly, by dividing the desired filtering between second primary(H-field) IF filter 6431 and second secondary (H-field) IF filter 6451,a narrower passband can be achieved with greater stability and greaterreliability than with a single filter stage alone. Second automatic gaincontrol 6481 preferably includes a received signal strength indicator(RSSI) and conveys an RSSI level to RSSI output 6221.

Third (E-field) antenna 632 is responsive to the electric or E-fieldcomponent of electromagnetic signal 615 and presents a received signalproportional to the electric or E-field component of electromagneticsignal 615 to third (E-field) pre-select filter 6402.

Third (E-field) pre-select filter 6402 passes a third representativesignal proportional to the electric or E-field component ofelectromagnetic signal 615, but rejects signals with undesirablefrequencies. Third (E-field) pre-select filter 6402 may be, for example,a band pass filter or a low pass filter. Typically third (E-field)pre-select filter 6402 will pass those frequencies within which beacon610 might transmit an electromagnetic signal 615 for a relevantapplication. Selection of a band will depend upon a variety of factorsincluding, but not necessarily limited to, regulatory constraints,propagation behavior of electromagnetic signal 615, and a desired ranger of operation.

Third (E-field) mixer 6422 mixes the third representative signalreceived from third (E-field) pre-select filter 6402 with a localoscillator (LO) signal generated by local oscillator 650 to generate athird intermediate frequency (or IF) representative signal. Localoscillator 650 may be a traditional sine wave oscillator, a directdigital synthesizer (DDS), or other oscillator or waveform templatesource.

Third primary (E-field) IF filter 6432 accepts only the desired third IFrepresentative signal and rejects other undesired signals. A crystalfilter may be advantageously used as third primary (E-field) IF filter6432. Such a crystal filter is characterized by an extremely narrow passband, and preferably has a constant group delay within the pass band. Anarrow pass band acts to allow the desired third IF representativesignal to be conveyed to third primary (E-field) IF amplifier 6442 whilerejecting adjacent undesired signals. Third primary (E-field) IFamplifier 6442 increases the amplitude of the third IF representativesignal and conveys the amplified third IF representative signal to thirdsecondary (E-field) IF filter 6452. Third secondary (E-field) IF filter6452 accepts only the desired third IF representative signal and rejectsother undesired signals A crystal filter may be advantageously used asthird secondary (E-field) IF filter 6452. Such a crystal filter ischaracterized by an extremely narrow pass band, and preferably has aconstant group delay within the pass band. A narrow pass band acts so asto allow the desired third IF representative signal to be conveyed tothird secondary (E-field) IF amplifier 6462 while rejecting adjacentundesired signals. Third secondary (E-field) IF amplifier 6462 increasesthe amplitude of the third IF representative signal and conveys thethird IF representative signal to signal output port 6212 and to thirdautomatic gain control (AGC) 6482.

Third automatic gain control 6482 adjusts a gain of second primary(E-field) IF amplifier 6442 and third secondary (E-field) IF amplifier6462 to maintain a desired level of the third IF representative signal.By dividing a desired total gain between third primary (E-field) IFamplifier 6442 and third secondary (E-field) IF amplifier 6462, a hightotal gain and a large dynamic range can be maintained with greaterstability and reliability than in a single amplification stage alone.Similarly, by dividing the desired filtering between third primary(E-field) IF filter 6432 and third secondary (E-field) IF filter 6452, amore narrow passband can be achieved with greater stability and greaterreliability than with a single filter stage alone. Third automatic gaincontrol 6482 preferably includes a received signal strength indicator(RSSI) and conveys an RSSI level to RSSI output 6222.

Local oscillator 650 may also be advantageously used as a tuner toselect among a plurality of electromagnetic signals 615 transmitted by aplurality of beacons 610. A particular beacon 610 emitting a particularelectromagnetic signal 615 may be distinguished from other beacons 610emitting other electromagnetic signals 615 with slightly differentfrequencies. Thus a single locator 620 may track a large number ofdifferent beacons 610. A variety of other schemes for tracking multiplebeacons 610 are possible, including, for example, time division multipleaccess. If a beacon 610 modulates a transmitted electromagnetic signal615 with identifying information, one can distinguish among a pluralityof beacons 610 operating at the same frequency. Similarly, a largenumber of different locators 620 may measure ranges r to a common beacon610.

Although synchronization is not required between beacon 610 and locator620, a common local oscillator 650 acts to maintain synchronizationamong a plurality of channels 625, 626, 627 within a single locator 620.Synchronization among plurality of channels 625, 626, 627 within locator620 is advantageous to enable precision phase comparisons among signalsreceived by plurality of channels 625, 626, 627.

In other embodiments, local oscillator 650 may tune a first channel 625,a second channel 626, or a third channel 627 (or various combinations ofchannels 625, 626, 627) to sweep through a variety of frequencies ofinterest. Micro-processor 692 may monitor and compile data from RSSIports 6220, 6221, 6222 (or various combinations of RSSI ports 6220,6221, 6222) to characterize a noise and interference environment.Micro-processor 692 may convey appropriate control signals through databus 695 to a plurality of beacons 610 to select optimal frequencies ormodes of operation given a characterized noise and interferenceenvironment. Similarly, in a dense signal environment with manysimultaneously operating beacons 610, micro-processor 692 may monitorsignals and convey appropriate control signals through data bus 695 to aplurality of beacons 610 to assign optimal frequencies or modes ofoperation among a plurality of beacons 610 for facilitating coexistencewithin and among the plurality of beacons 610. Further, micro-processor692 may monitor range r and convey appropriate control signals throughdata bus 695 to a respective beacon 610 to assign an optimal frequencyor mode of operation appropriate for the respective beacon 610appropriate for operation at a detected range r to the respective beacon610.

In other embodiments, channels in addition to channels 625, 626, 627 maybe used so that a locator 620 may simultaneously track a plurality ofbeacons 610 generating electromagnetic signals 615 at differentfrequencies. Further, additional channels may be advantageously employedin detecting and characterizing a noise and interference environment. Instill other embodiments, additional channels associated with alternatepolarizations may enable ranging system 600 to make measurementsunimpaired by the relative orientation of a beacon 610 with respect to alocator 620.

In ranging system 600 (FIG. 6), first phase detector 681 receives thefirst IF representative signal from first signal output port 6210 andreceives the third IF representative signal from third signal outputport 6212 and determines phase difference between the first and third IFrepresentative signals. Second phase detector 682 receives the second IFrepresentative signal from second signal output port 6211 and receivesthe third IF representative signal from third signal output port 6212and determines phase difference between the second and third IFrepresentative signals. In a preferred embodiment, locator 620 has twoH-field channels (first (H-field) channel 625 and second (H-field)channel 626) and a third (E-field) channel 627. In a preferredembodiment using a vertically polarized electromagnetic signal 615,third electric antenna 632 is a vertical whip antenna with anomni-directional pattern in a first plane perpendicular to the axis ofthe whip. In a preferred embodiment magnetic antennas 630, 631 are loopantennas with an omni-directional pattern in a second planesubstantially perpendicular to a first plane (associated with the whipantenna of third electric antenna 632). It is advantageous to have twomagnetic antennas 630, 631 to achieve sensitivity to a magneticcomponent of an electromagnetic signal 615 incident in any direction.With only one magnetic antenna 630 or 631 locator 620 will tend beinsensitive to a beacon 610 positioned in a direction that lies in anull of the single magnetic antenna 630 or 631. By having two magneticantennas 630, 631 locator 620 can determine range r to a beacon 610 inany direction. An additional advantage of having two magnetic antennas630, 631 is that locator 620 may use prior art techniques to obtainangle of arrival information in addition to range information.

For optimal performance of phase detectors 681, 682 it is advantageousfor amplitudes of first, second and third IF representative signals tobe maintained within a desired amplitude limit. Automatic gain controls6480, 6481, 6482 act to maintain a desired amplitude limit for thefirst, second and third IF representative signals. Phase detectors 681,682 may employ log amps to maintain constant signal levels, such as areused in an Analog Devices part no. AD 8302 (phase detector IC).Alternatively, channels 625, 626, 627 may include a limiter (not shownin FIG. 6) to limit output signal levels.

Range detector 690 translates measured phase differences received fromphase detectors 681, 682 to range r. In a preferred embodiment, rangedetector 690 includes an analog to digital converter 691 and amicroprocessor (or a micro-controller) 692 that cooperate to calculaterange r based upon signals received from one or both of phase detectors681, 682. In a preferred embodiment, range detector 690 also monitorsRSSI levels from RSSI ports 6220, 6221, 6222 so that range detector 690can select either of phase detectors 681, 682 (or both) to use indetermining range r. Range detector 690 may also compare RSSI levelsfrom RSSI ports 6220, 6221, 6222 to determine angle of arrival ofelectromagnetic signal 615. Typically first phase detector 681 will bepreferred if beacon 610 lies in the pattern of first magnetic fieldantenna 630 and second phase detector 682 will be preferred if beacon610 lies in the pattern of second magnetic field antenna 631. Ideallyrange detector 690 will selectively employ signals received from phasedetectors 682, 682 to optimize range measurement. Such optimizationmight also involve, for example, locator 620 combining signals receivedfrom magnetic field antennas 630, 631 to create an effective antennapattern that nulls out an interfering signal, or maximizes a desiredsignal. RSSI levels from RSSI ports 6220, 6221, 6222 may also be used byrange detector 690 to supplement or complement information from phasedetectors 681, 682 in determining range r.

Range detector 690 may include visual, audio, or other output formats toindicate range r to a user, or may convey a measured range r to a remotelocation for further analysis as part of a comprehensive positioning,tracking, or locating system. Range detector 690 may also include meansto control local oscillator 650 including (but not necessarily limitedto) setting a frequency of a local oscillator signal.

Data bus 695 is optional and when employed allows data and controlsignals to be conveyed between locator 620 and beacon 610. Data bus 695may involve a wireless network (such as an 802.11b network), a hardwired network (such as an Ethernet connection or a serial cable), or mayemploy modulation of electromagnetic signal 615 transmitted by beacon610. A plurality of locators 620 and beacons 610 may share a common databus 695. Such a plurality of locators 620 and beacons 610 may operatecooperatively to establish a comprehensive tracking, positioning, orlocating system. With a wireless data bus 695, beacon 610 is no longerstrictly a transmit-only device. Because only a transmittedelectromagnetic signal 615 is necessary for ranging operations, with awireless data link precise timing required for a traditional transponderranging system is eliminated. Timing information can be conveyed via thewireless data link.

Locator 620 may be regarded as comprising a means for detecting orreceiving a first (H-field) signal, a means for detecting or receiving asecond (H-field) signal, a means for detecting or receiving a third(E-field) signal, a means for determining a first phase differencebetween a first and a third signal, a means for determining a secondphase difference between a second and a third signal, and a means fordetermining a range r given a first and a second phase difference. Itmay also be advantageous to include in locator 620 a means for tuning alocator 620 whereby range data may be obtained for any of a plurality ofbeacons 610, each generating an electromagnetic signal at a differentfrequency.

Still further advantages may accrue by adding to locator 620 a means forconveying data among a plurality of locators 620 and a plurality ofbeacons. Such a means (e.g., a data bus or a wireless link 695) could beadvantageously employed in a comprehensive tracking, positioning, orlocating system.

It should be kept in mind that functions and components of locator 620need not be implemented in a single unit. For example, it may beadvantageous to place first (H-field) antenna 630, second (H-field)antenna 631, and third (E-field) antenna 632 at respective locationsdistant from other components or functionality of locator 620. Antennasmay, for example, be connected via RF cables if a stand-off were desiredfor safety reasons, economic reasons, operational reasons, ease-of-useor for any other reasons. Similarly, locator 620 may implement signaldetection and reception in one location and phase detection in another.Locator 620 may also implement phase detection in one location and relaydata to a range detector 690 at a remote location for determination ofrange r.

Combined Beacon-Locator

FIG. 7 is a schematic diagram of a system for near-field ranging bycomparison of electric and magnetic field phase with beacon and locatorfunction combined in a single unitary device. In FIG. 7, a combinedbeacon-locator apparatus 700 is configured to operate as a beacon whoserange r from a remote locator (such as a remote beacon-locator apparatus710 operating as a locator) may be measured by the remote locator.Alternatively, beacon-locator apparatus 700 can operate as a locatorthat measures range r to another beacon (such as remote beacon-locatorapparatus 710 operating as a beacon).

Beacon-locator apparatus 700 includes a first magnetic (H-field) antenna730, a second (E-field) antenna 732, a transmit-receive switch 728, atransmitter 712, and a locator receiver 720. Locator receiver 720includes a first (H-field) receiver 722, a second (E-field) receiver742, a phase detector 781, and a range detector 790. An optional databus 795 permits communication between or among a plurality ofbeacon-locators, beacons, locators, or other devices.

Combined Beacon-Locator in Locator Mode

Remote beacon-locator apparatus 710 (operating in a bacon mode)transmits an electromagnetic signal 715 that is received bybeacon-locator system 700 operating in a locator mode. First (H-field)antenna 730 is sensitive to a magnetic component of an incidentelectromagnetic signal 715 and conveys a representative magnetic signalproportional to the magnetic component of electromagnetic signal 715 toan antenna port 7270 of first (H-field) receiver 722.

First (H-field) receiver 722 receives the representative magnetic signalat first antenna port 7220, and receives a local oscillator (LO) signalfrom a local oscillator 750 at a local oscillator port 7230. Usingfiltering, amplification and mixing means generally known topractitioners of the RF arts (an example of which is described inconnection with FIG. 6), first (H-field) receiver 722 presents a firstreceived intermediate frequency (IF) representative signal at a firstoutput port 7210 and an RSSI signal at an RSSI port 7220.

Because beacon-locator apparatus 700 is operating in a locator mode,transmit-receive switch 728 is set to couple second (E-field) antenna732 to second (E-field) receiver 742. In an alternate embodiment,transmit-receive switch 728 may be a circulator or other device thatallows a beacon-locator, such as beacon-locator apparatus 700, tofunction as a beacon and as a locator simultaneously. Second (E-field)antenna 732 sensitive to the electric component of incidentelectromagnetic signal 715 and conveys a representative electric signalproportional to the electric component of electromagnetic signal 715 toan antenna port 7271 of second (E-field) receiver 742.

Second (E-field) receiver 742 receives the representative electricsignal at second antenna port 7271, and receives a local oscillator (LO)signal from local oscillator 750 at a local oscillator port 7231. Usingfiltering, amplification and mixing means generally known topractitioners of the RF arts (an example of which is discussed inconnection with FIG. 6), second (E-field) receiver 742 presents a secondreceived intermediate frequency (IF) representative signal at a secondoutput port 7211 and an RSSI signal at an RSSI port 7221.

Phase detector 781 receives the first representative signal from outputport 7210 and receives the second representative signal from output port7211. Phase detector 781 generates a phase difference output signalproportional to the phase difference between the first and secondrepresentative signals and conveys the phase difference output signal torange detector 790.

Range detector 790 includes an analog to digital converter 791 and amicro-processor 792. Range detector 790 receives RSSI signals from RSSIports 7220, 7221 and the phase difference output signal from a phasedetector 781. Analog to digital converter 791 converts these signals todigital signals and conveys them to micro-processor 792. Micro-processor792 calculates range r based upon the digital signal inputs receivedfrom analog to digital converter 791. Among the means by which amicro-processor 792 may determine a range r are, for example: 1) Freespace theory as presented in Equation [8], 2) a more precise analyticalor numerical model including ground and other effects of a propagationenvironment, and 3) a model based upon empirical measurements. Range rmay be calculated from a phase input alone or using a more complicatedmodel including input from RSSI ports 7220, 7221.

Micro-processor 792 may adjust a frequency of local oscillator 750 totune first (H-field) receiver 722 and second (E-field) receiver 742.This enables beacon-locator apparatus 700 to measure range r of avariety of other beacons 710 or beacon-locators 700 operating atdifferent frequencies. Micro-processor 792 also enables beacon-locatorapparatus 700 to use a frequency hopping system or power control schemefor added security and robustness.

Micro-processor 792 may have a user interface means such as an audio orvisual display to provide a user with a range measurement. In additionmicro-processor 792 may convey range or other information to anotherlocation via an optional data bus 795 as part of a comprehensive systemthat relies on tracking or positioning input, or for another purpose.

Exemplary beacon-locator apparatus 700 has two channels, first (H-field)receiver channel 722 and second (E-field) receiver channel 742.Additional channels may be preferred if better performance is desired atthe cost of additional complexity and expense. Such additional channelscould be used to detect E-field and H-field components in alternatepolarizations including but not limited to polarization componentslongitudinal to a direction of an incident electromagnetic signal 715.Thus beacon-locator apparatus 700 could be less dependent upon aparticular orientation of an incident electromagnetic signal 715 andthereby offer more robust performance. These same benefits also accruefor locators that are not combined with beacons to form beacon-locators.

Combined Beacon-Locator in Beacon Mode

When beacon-locator apparatus 700 operates in a beacon mode,micro-processor 792 triggers transmit-receive switch 728 to connecttransmitter 712 to antenna 732. Micro-processor 792 also sets anappropriate frequency for a transmitter 712. Exemplary beacon-locatorapparatus 700 uses electric antenna 732 as a beacon transmit antenna.Magnetic antenna 730 could just as readily be used. The choice ofantenna to be used for transmission operation in a beacon mode dependsupon several factors including, for example, pattern, performance inproximity of other objects, polarization, matching, and propagationenvironment.

Remote beacon-locator apparatus 710 includes an electric antenna 735 anda magnetic antenna 733. Transmitter 712 sends an RF signal to transmitantenna 732. Transmit antenna 732 radiates an electromagnetic signal 716that is received by electric antenna 735 and by magnetic antenna 733when remote beacon-locator apparatus 710 operates in a locator mode.Remote beacon-locator apparatus 710 receives an H-field signal frommagnetic antenna 733 and receives an E-field signal from electricantenna 735 thus allowing remote beacon-locator apparatus 710 todetermine range r to beacon-locator apparatus 700.

An optional data bus 795 allows beacon-locator apparatus 700 to interactand coordinate with remote beacon-locator apparatus 710. For example,beacon-locator apparatus 700 can trigger remote beacon-locator apparatus710 to cause remote beacon-locator apparatus 710 to transmit and allowbeacon-locator apparatus 700 to determine range r to remotebeacon-locator apparatus 710. An appropriate trigger might, for example,include data regarding a communication frequency, a frequency-hoppingpattern, power control feedback or another characteristics of a transmitsignal to be radiated from remote beacon-locator apparatus 710. Atrigger might further include identification or authenticationinformation.

Transmitter 712 may be controlled by micro-processor 792 to modulateelectromagnetic signal 716 with information. A wide variety ofmodulation techniques are possible. Binary phase shift key (BPSK) is onepreferred modulation option. BPSK is advantageous because of itssimplicity. Further, because the present invention relies on a relativedifference between electric and magnetic field phases, a common modephase shift (such as happens with BPSK and similar modulations) does noteffect the ability of the present invention to measure range r. Suchinformation may include identifying or authentication information, orother information or telemetry of value to a user.

Antenna Configurations

FIGS. 8-11 reveal a variety of antenna configurations for rangingsystems 800, 900, 1000, 1100. FIG. 8 is a schematic illustration of arepresentative antenna configuration for a near-field ranging systemhaving a vertical polarization beacon and a vertical polarizationomni-directional locator. In FIG. 8, ranging system 800 includes avertical polarization beacon 810 and locator 820. A verticalpolarization antenna 836 associated with vertical polarization beacon810 is typically a vertically oriented whip or dipole antenna, but couldbe a loop or loopstick antenna oriented to radiate vertically polarizedelectromagnetic signals 815 in a desired direction. In many applicationsan omni-directional coverage of a single vertically oriented whip ispreferred to a more directional pattern of a traditional verticallypolarized loop. Locator 820 includes an electric antenna 832, a firstmagnetic antenna 831, and a second magnetic antenna 833 orientedperpendicularly to first magnetic antenna 831. Electric antenna 832 istypically a vertically oriented whip or dipole antenna. First magneticantenna 831 and second magnetic antenna 833 are typically loop orloopstick antennas oriented to be responsive to vertically polarizedelectromagnetic signal 815. Locator 820 can select either first magneticantenna 831 or second magnetic antenna 833 to optimize a received(H-field) signal. Locator 820 may also use signals from both firstmagnetic antenna 831 and second magnetic antenna 833.

FIG. 9 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization omni-directionallocator. In FIG. 9, ranging system 900 includes a horizontalpolarization beacon 910 and locator 920. A horizontal polarizationantenna 937 associated with horizontal polarization beacon 910 istypically a vertically oriented loopstick or loop antenna oriented in ahorizontal plane, but could be a whip or dipole antenna oriented toradiate horizontally polarized electromagnetic signals 915 in a desireddirection. In many applications the omni-directional coverage of asingle loop or loopstick antenna is preferred to a more directionalpattern of a traditional horizontally polarized whip or dipole antenna.Locator 920 includes a magnetic antenna 931, a first electric antenna932, and a second electric antenna 934. Magnetic antenna 931 istypically a vertically oriented loopstick or loop antenna oriented in ahorizontal plane. First electric antenna 932 and second electric antenna934 are typically dipole or whip antennas oriented to be responsive tohorizontally polarized electromagnetic signals 915. Locator 920 canselect either first electric antenna 932 or second electric antenna 934to optimize a received (E-field) signal. Locator 920 may also usesignals from both first electric antenna 932 and second electric antenna934.

FIG. 10 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a verticalpolarization beacon and a vertical polarization directional locator. InFIG. 10, ranging system 1000 includes a vertical polarization beacon1010 and locator 1020. A vertical polarization antenna 1036 associatedwith vertical polarization beacon 1010 is typically a verticallyoriented whip or dipole antenna oriented in a vertical plane, but couldbe a loop or loopstick antenna oriented to radiate vertically polarizedelectromagnetic signals 1015 in a desired direction. In manyapplications the omni-directional coverage of a single verticallyoriented whip antenna is preferred to a more directional pattern of atraditional vertically polarized loop antenna. Locator 1020 includes anelectric antenna 1032 and a magnetic antenna 1031. Electric antenna 1032is typically a vertically oriented whip or dipole antenna. Magneticantenna 1031 is typically a loop or loopstick antenna oriented to beresponsive to vertically polarized electromagnetic signals 1015. Locator1020 typically must be oriented to optimize a signal from magneticantenna 1031. Additionally, the direction of arrival of electromagneticsignal 1015 can be determined by orienting a null of magnetic antenna1031 with the direction of arrival of electromagnetic signal 1015 andobserving an associated decrease in an RSSI level. If the responses ofmagnetic antenna 1031 and electric antenna 1032 are summed, thedirection of arrival of electromagnetic signal 1015 can be determined byorienting a null of an effective summed pattern with a direction ofarrival of electromagnetic signal 1015 and observing an associateddecrease in amplitude of the summed responses.

FIG. 11 is a schematic illustration of a representative antennaconfiguration for a near-field ranging system having a horizontalpolarization beacon and a horizontal polarization directional locator.In FIG. 11, ranging system 1100 includes a horizontal polarizationbeacon 1110 and locator 1120. A horizontal polarization antenna 1137associated with horizontal polarization beacon 1110 is typically aloopstick antenna oriented vertically or a loop antenna oriented in ahorizontal plane, but could be a whip or dipole antenna oriented toradiate horizontally polarized electromagnetic signals 1115 in a desireddirection. In many applications the omni-directional coverage of asingle loop or loopstick antenna is preferred to a more directionalpattern of a traditional horizontally polarized whip or dipole antenna.Locator 1120 includes an electric antenna 1132 and a magnetic antenna1131. Electric antenna 1132 is typically a horizontally oriented whip ordipole antenna. Magnetic antenna 1131 is typically a loop or loopstickantenna oriented to be responsive to horizontally polarizedelectromagnetic signals 1115. Locator 1120 typically must be oriented tooptimize a signal from electric antenna 1132. Additionally, thedirection of arrival of electromagnetic signal 1115 can be determined byorienting a null of electric antenna 1132 with the direction of arrivalof electromagnetic signal 1115 and observing an associated decrease inan RSSI level. If the responses of magnetic antenna 1131 and electricantenna 1132 are summed, the direction of arrival of electromagneticsignal 1115 can be determined by orienting a null of an effective summedpattern with a direction of arrival of electromagnetic signal 1115 andobserving an associated decrease in amplitude of the summed responses.

A choice of polarization may be influenced by specifics of a particularpropagation environment, by the presence of potentially interferingsignals of a particular polarization, or by the requirements of aparticular application. Vertical polarization is typically preferred forpropagation in an environment where undesired coupling tends to behorizontal, such as near ground. Horizontal polarization is typicallypreferred for propagation in an environment where undesired coupling isvertical such as through vertically oriented steel members. Circularpolarization is typically preferred for systems where orientationindependence is important. Some such coupling may actually be desirableif this coupling tends to guide waves in a desired direction.

Important antenna parameters for designing ranging systems according tothe present invention include antenna patterns, matching, form factors,performance and cost. Another important critical parameter is capturingand differentiating between an electric and a magnetic component of anincident electromagnetic signal. A wide variety of suitable antennaoptions are known to those skilled in the RF arts.

Exemplary Receiver

The inventors have implemented a ranging system as taught by the presentinvention. This system operated at 10.7 MHz and exhibited rangingaccuracies within inches from about 5 ft to about 35 ft. Since thewavelength (λ) at 10.7 MHz is 92 ft, this corresponds to about 0.054 λto 0.38 λ. According to the teachings of the present invention,significantly longer ranges are possible by utilizing significantlylower frequencies.

FIG. 12 is a schematic diagram illustrating details of an exemplaryreceiver in a system for electromagnetic ranging. In FIG. 12, a rangingsystem 1200 includes a beacon 1210 and a locator 1220. Beacon 1210transmits an electromagnetic signal 1215 that is received by locator1220. Locator 1220 includes an electric antenna 1232 that is sensitiveto the electric component of electromagnetic signal 1215. Electricantenna 1232 detects a first (electric or E-field) signal proportionalto the electric component of electromagnetic signal 1215 and conveys thefirst signal to an antenna port 1270 of a first receiver 1225 includedin locator 1220. Locator 1220 also includes a magnetic antenna 1231 thatis sensitive to the magnetic component of electromagnetic signal 1215.Magnetic antenna 1231 detects a second (magnetic or H-field) signalproportional to the magnetic component of electromagnetic signal 1215and conveys the second signal to a second receiver 1227 included inlocater 1220. Second receiver 1227 is constructed in substantialsimilarity to receiver 1225; details of construction of receiver 1227are omitted in FIG. 12 in order to simplify the description of rangingsystem 1200.

Exact spacing between electric antenna 1232 and magnetic antenna 1231 isnot critical, providing that spacing is large enough to avoid undesiredmutual coupling and spacing is small relative to the wavelength λ ofelectromagnetic signal 1215. The inventors have arranged electricantenna 1232 and magnetic antenna 1231 separated by a distance on theorder of 1%-3% of a wavelength (0.03λ-0.01λ). In alternate embodiments,electric antenna 1232 and magnetic antenna 1231 may be arranged in asingle integral unit with a first terminal yielding an E-field responseand a second terminal yielding an H-field response. Although spacingbetween antennas is preferentially small relative to the wavelength λ ofelectromagnetic signal 1215, a larger spacing between electric antenna1232 and magnetic antenna 1231 may be tolerated if phase detector 1280or range detector 1290 in locator 1220 are compensated for the effect ofthe larger spacing.

Locator 1220 also includes a pre-select filter 1242 that receives thefirst (electric) signal from antenna port 1270. Pre-select filter 1242passes the first (electric) signal in a desired band, but rejectssignals with undesirable frequencies. Typically pre-select filter 1242will pass a band of frequencies within which beacon 1210 might transmitan electromagnetic signal 1215 for a relevant application. Selection ofa band will depend upon a variety of factors including, but notnecessarily limited to, regulatory constraints, propagation behavior ofan electromagnetic signal 1215, and a desired range r of operation. Thepresent invention offers optimal performance for a desired range r ofoperation approximately constrained by 0.08 λ to 0.30 λ, where λ is thewavelength of the electromagnetic signal 1215 transmitted by beacon1210. A typical operating range is generally within 0.05λ to 0.50λ.Higher performance implementations of the present invention may operateat ranges r less than 0.05λ and greater than 0.50λ.

A front-end-amplifier 1265 increases the amplitude of the first(electric) signal. If atmospheric and other noise are sufficiently low,it is advantageous for an amplifier to have a noise figure sufficientlylow to avoid introducing undesired noise, a dynamic range large enoughto accommodate the potential variation in amplitude of the first(electric) signal, and a gain sufficient to yield a suitably largeamplitude first (electric) signal so that a weak signal will properlydrive phase detector 1281. The inventors have advantageously used aMini-Circuits ZFL-500 amplifier as a front-end-amplifier 1265, but awide variety of other amplifiers are suitable.

A mixer 1252 mixes the first (electric) signal with a local oscillator(LO) signal generated by a local oscillator 1250 thus yielding a firstintermediate frequency (IF) signal. Local oscillator 1250 may be atraditional sine wave oscillator. Local oscillator 1250 may also be adirect digital synthesizer (DDS), or other waveform template generator.For instance, the inventors have used an Analog Devices DDS (AD 9835) aslocal oscillator 1250 and a Mini-Circuits SBL-3 mixer as mixer 1252. Awide variety of alternate implementations are possible.

An IF amplifier 1262 increases the amplitude of the first IF signal. Theinventors have found that a pair of current feedback operationalamplifiers providing about +50 dB of gain were a suitable embodiment ofIF amplifier 1262, but a wide variety of alternatives are available topractitioners of the RF arts.

An IF filter 1244 accepts only the desired first IF signal and rejectsother undesired signals. A crystal filter may be advantageously used asIF filter 1244. Such a crystal filter is characterized by an extremelynarrow pass band, and preferably has a constant group delay within thepass band. A narrow pass band acts so as to allow the desired first IFsignal to be conveyed to phase detector 1281 while rejecting adjacentundesired signals and noise.

Local oscillator 1250 may also be advantageously used as a tuner toselect among a plurality of electromagnetic signals transmitted by aplurality of beacons 1210. A particular beacon 1210 emitting aparticular electromagnetic signal may be distinguished from otherbeacons emitting other electromagnetic signals, where other signals haveslightly different frequencies. Thus a single locator 1220 may track alarge number of different beacons 1210. A variety of other schemes fortracking multiple beacons are possible, including for example, timedivision multiple access, code division multiple access, frequencyhopping, or other schemes for achieving a desired channelization.Similarly, a large number of different locators 1220 may measure rangesto a particular beacon 1210. Local oscillator 1250 may be considered asa component of an individual receiver 1225 or 1227 or as a commonfrequency standard for a plurality of receivers 1225, 1227.

Phase detector 1281 accepts the first IF signal from first receiver 1225and a second IF signal from second receiver 1227 and generates an outputvoltage proportional to a phase difference between the first IF signaland the second IF signal. For purposes of illustration and notlimitation, one exemplary embodiment of phase detector 1280 is an AnalogDevices AD 8302. This particular phase detector also yields an outputproportional to a magnitude difference that may help identify andcorrect for propagation anomalies and provide a more accuratedetermination of range in some circumstances.

Range detector 1290 is included in locator 1220 and accepts an inputfrom a phase detector 1281 for determining range r between beacon 1210and locator 1220. The inventors used a Measurement Computing CorporationPC-Card-DAS 16/16 A/D PCMCIA Card and a notebook computer to embodyrange detector 1290, but there are a great many ways one skilled in theRF arts could implement range detector 1290.

The present invention offers good performance for a desired range ofoperation approximately within ranges r between 0.05 λ and 0.50 λ away,and more optimal performance was achieved within a range r between 0.08λ and 0.30 λ where λ is the wavelength of electromagnetic signal 1215transmitted by beacon 1210. Higher performance implementations of thepresent invention may operate at ranges r less than 0.05λ and greaterthan 0.50λ.

Fixed Beacon-Mobile Locator Architecture

FIG. 13 is a schematic diagram illustrating a near-field ranging systemconfigured according to a fixed beacon-mobile locator architecture. InFIG. 13, a fixed beacon-mobile locator ranging system 1300 includes afirst beacon 1310 in a first known, fixed position transmitting a firstelectromagnetic signal 1315. A locator 1320 receives firstelectromagnetic signal 1315 and determines a first range r₁. A secondbeacon 1312 in a second known, fixed position transmits a secondelectromagnetic signal 1317. Locator 1320 receives secondelectromagnetic signal 1317 and determines a second range r₂. A thirdbeacon 1314 in a third known, fixed position transmits a thirdelectromagnetic signal 1319. Locator 1320 receives third electromagneticsignal 1319 and determines a third range r₃. A fourth beacon 1316 in afourth known, fixed position transmits a fourth electromagnetic signal1321. Locator 1320 receives fourth electromagnetic signal 1321 anddetermines a fourth range r₄. Electromagnetic signals 1315, 1317, 1319,1321 may be substantially similar electromagnetic signals withsubstantially similar frequencies, or may be a variety ofelectromagnetic signals with different frequencies. Electromagneticsignals 1315, 1317, 1319, 1321 may be transmitted substantiallycontemporaneously or at different times. For example, beacon 1310 maysimultaneously transmit a low frequency signal suitable for a long rangeand a high frequency signal suitable for a short range. Using ranges r₁,r₂, r₃, r₄, locator 1320 can determine its position. For purposes ofexplanation and not for limitation, four beacons 1310, 1312, 1314, 1316have been illustrated. One beacon is sufficient to yield useful rangeinformation for some applications. Two beacons can yield a position intwo dimensions subject to an ambiguity, three beacons can yield anunambiguous position in two dimensions or an ambiguous position in threedimensions, and four beacons yield an unambiguous position in threedimensions. With additional beacons providing ranges, one can obtain amore accurate position for locator 1320 using multilateration techniquesknown to those skilled in the RF arts.

Locator 1320 can also convey range and other useful information via anoptional data bus 1395 to a central controller 1399 for analysis.Central controller 1399 can then relay position or other information viadata bus 1395 back to locator 1320. A centrally coupled (i.e., coupledto all components of ranging system 1300) controller 1399 or locator1320 can coordinate frequency of operation or other operationalparameters of locator 1320 and beacons 1310, 1312, 1314, 1316. Suchcoordination may include operating at appropriate frequencies to avoidinterference or to obtain optimal range information. Coordination mayalso include scheduling time or duty cycle of operation. Coordinationmay further include control of transmit power for coexistence, signalsecurity, or other reasons.

Fixed beacon-mobile locator system 1300 is advantageous when one wishesto track a limited number of assets, or if one wishes position,location, navigation, or guidance information to be available at apotentially large number of mobile locations. Fixed beacon-mobilelocator system 1300 is suitable for providing a user (with a locator1320) with fast updates of position within an area around or throughoutwhich a plurality of beacons (e.g., beacons 1310, 1312, 1314, 1316) havebeen deployed. A variety of applications are possible. For purposes ofillustration and not for limitation, a few applications are listedbelow.

For example, fixed beacons 1310, 1312, 1314, 1316 may be deployed in andaround a golf course, a lawn, a farm, or another area in which precisionguidance of equipment is desired. Locator 1320 may be placed on arobotic tractor, mower, golf ball gatherer, harvester, fertilizer, orother equipment. Locator 1320 may be used in a guidance or navigationsystem for such equipment. Locator 1320 may also be used to keep trackof golf carts, or other assets. Locator 1320 may be used to assistgolfers or others in determining their location and in particular theirlocation relative to a golf hole or another landmark of interest.

Fixed beacons 1310, 1312, 1314, 1316 may be deployed in and around amall, store, museum, business, amusement park, urban area, park,wilderness area, harbor, lake, property, home, apartment or another areaor facility in which one wishes individuals or equipment to be able tomonitor their location or position. Locator 1320 may be carried by anindividual so that an individual may monitor his or her own location ora location of another individual (such as a family member, friend, orother individual of interest). Locator 1320 may also be carried by anindividual so that an individual may determine their location relativeto a landmark or other point or points of interest. Locator 1320 may beincorporated in a device that provides a user with location-specificinformation such as a price or other information pertinent to a nearbyobject for sale, review, or evaluation. Locator 1320 may be incorporatedin a device that provides a user with location-specific informationdescribing a nearby attraction, display, exhibit, hazard, or otherfeature of potential interest.

Locator 1320 may be incorporated into a vehicle to provide position,guidance, or navigation information. An example is a precision guidanceor navigation system for aircraft such as unmanned aerial vehicles(UAV), boats, automobiles, unmanned ground vehicles (UGV) or othervehicles.

Fixed/Mobile Locator-Mobile Beacon Architecture

FIG. 14 is a schematic diagram illustrating a near-field ranging systemconfigured according to a fixed/mobile locator-mobile beaconarchitecture. In FIG. 14, a fixed/mobile locator-mobile beacon rangingsystem 1400 includes a mobile beacon 1410 transmits a firstelectromagnetic signal 1415, a second electromagnetic signal 1417, athird electromagnetic signal 1419, a fourth electromagnetic signal 1421,and a fifth electromagnetic signal 1423. Electromagnetic signals 1415,1417, 1419, 1421, 1423 may be substantially similar electromagneticsignals with substantially similar frequencies, or a variety ofelectromagnetic signals with different frequencies. Electromagneticsignals 1415, 1417, 1419, 1421, 1423 may be transmitted at asubstantially similar time or at different times. For example, mobilebeacon 1410 may simultaneously transmit a low frequency signal suitablefor a long range and a high frequency signal suitable for a short range.

A first fixed locator 1420 receives first electromagnetic signal 1415and determines a first range r₁. A second fixed locator 1422 receivessecond electromagnetic signal 1417 and determines a second range r₂. Athird fixed locator 1424 receives third electromagnetic signal 1419 anddetermines a third range r₃. A fourth fixed locator 1426 receives fourthelectromagnetic signal 1421 and determines a fourth range r₄. A fifthmobile locator 1428 receives fifth electromagnetic signal 1423 anddetermines a fifth range r₅. For purposes of illustration, fifth mobilelocator 1428 is shown as a directional locator of the sort described asdirectional locator 1020 (FIG. 10), but fifth mobile locator 1428 couldas readily be an omni-directional locator of the sort described asomni-directional locator 820 (FIG. 8).

For purposes of explanation and not for limitation, four fixed locators1420, 1422, 1424, 1426 and one mobile locator 1428 are illustrated inFIG. 14. A single locator is sufficient to yield useful rangeinformation for some applications. For example, a single mobile locator1428 can enable a user to ascertain range r₅ from mobile beacon 1410,thus allowing the user to home in on mobile beacon 1410. Two locatorscan yield a position in two dimensions subject to an ambiguity, threelocators can yield an unambiguous position in two dimensions or anambiguous position in three dimensions, and four locators yield anunambiguous position in three dimensions. With additional locatorsproviding ranges one can obtain a more accurate position for beacon 1410using multilateration techniques known to those skilled in the RF arts.

When a data bus 1495 is included in ranging system 1400, locators 1420,1422, 1424, 1426, 1428 may transmit ranges r₁, r₂, r₃, r₄, r₅ via databus 1495 to a central controller 1499 or another device (not shown inFIG. 14) connected to data bus 1495. Central controller 1499 can gatherranges r₁, r₂, r₃, r₄, r₅, calculate a position of beacon 1410, andrelay that position information to any other device connected to databus 1495.

Central controller 1099 (or another device connected to data bus 1495)can coordinate a frequency of operation or other operational parametersof mobile beacon 1410 and locators 1420, 1422, 1424, 1426, 1428. Suchcoordination may include operating at appropriate frequencies to avoidinterference or to obtain optimal range information. Coordination mayalso include scheduling time or duty cycle of operation. Coordinationmay further include control of transmit power for coexistence, signalsecurity, or other reasons.

Ranging system 1400 is particularly well configured for tracking largenumbers of assets including, for example, tracking people or assets froma central location. A variety of applications are possible. For purposesof illustration and not for limitation, a few applications are listedbelow.

For example, a plurality of fixed locators (e.g., locators 1420, 1422,1424, 1426) may be deployed in and around a particular area of interestwithin which one wishes to track a plurality of beacons (e.g., beacon1410) attached to assets of interest. Ranging system 1400 is well suitedfor tracking cars, rental equipment, parts, components, tools or otherassets in a manufacturing facility, a retail lot, warehouse, hold,vehicle, cargo container, storage area, hospital, or other facility inwhich one desires to track assets. A respective mobile beacon 1410 maybe placed in each car, piece of rental equipment, part, component, tool,or other asset whose location is desired to be known. If a respectivemobile beacon 1410 is removed from an area in and around which aninfrastructure of fixed locators have been placed, then a mobile locator(e.g., mobile locator 1428) may be used to help locate the wanderingmobile beacon 1410. This functionality is of particular utility if awandering mobile beacon 1410 is attached to stolen property. A locatorsuch as locator 1420 may be associated with a traffic signal, tollbooth, or other traffic related infrastructure and may monitor arespective mobile beacon 1410 in an approaching emergency vehicle, bus,or car thus allowing precision control of a traffic signal, or othermonitoring of the situation. It is useful to note here thatelectromagnetic signals associated with ranging system 1400 may bemodulated to include information, such as identifying informationrelating to an asset to which a mobile beacon is attached. In suchmanner, various assets bearing respective mobile beacons 1410 may beindividually identified or authenticated within ranging system 1400.

Further, a plurality of fixed locators (e.g., locators 1420, 1422, 1424,1426) may be deployed in and around a particular area of interest withinwhich one wishes to track a plurality of beacons (e.g., beacon 1410)attached or associated with people. Thus, ranging system 1400 is wellsuited for tracking emergency responders such as firefighting, police,SWAT team members, and medical personnel at an incident scene. Rangingsystem 1400 can be used to track employees in a hazardous environmentlike miners in a mine, workers at a facility where hazardous materialsare present, or corrections officers or prisoners in a prison. Rangingsystem 1400 may also be used to track patients, doctors, or other keypersonnel or equipment in a hospital, nursing home, or otherinstitution.

In still another exemplary application, ranging system 1400 may trackskiers at a ski area, allowing skiers to be readily located even in caseof an avalanche or other emergency. Similar applications includetracking hikers, climbers, skydivers, hunters, fishermen, outdoorsmen,and others who engage in potentially dangerous activities and mightrequire rescue or assistance.

Patrons may be tracked at an amusement park, museum, festival, sportingevent, convention, meeting, or other assembly drawing crowds. Sportscompetitors such as football players, soccer players, baseball players,swimmers, runners, and participants in other sports may have theirpositions monitored to assist in officiating, coverage, or analysis of asporting event. Sporting equipment or animals might be tracked,including, by way of example and not by way of limitation, footballs,baseballs, soccer balls, rugby balls, race cars, yachts, thoroughbreds,or greyhounds.

Key personnel may be located in a business or other facility. Childrenand others requiring supervision may be monitored around a home,neighborhood, school, campus, or other facility. Ranging system 1400 isalso applicable to a personal emergency response system (PERS), allowingrescuers to quickly locate an individual in need of assistance, such asa patient who has wandered away from a nursing home. Prisoners may betracked as part of a home release or other low security supervisionprogram. Persons subject to restraining orders or other restrictions ontheir movements may be monitored to prevent their violating terms oftheir restrictions. A mobile locator (e.g., mobile locator 1428) can beused to help find a person who has left an area in and around which aninfrastructure of fixed locators (e.g., fixed locators 1420, 1422, 1424,1426) have been placed.

Ranging system 1400 may also be used to track a pet as part of a petcontainment system, or to allow an owner to monitor a pet's location.Wildlife may be tracked as part of a conservation project, researcheffort, or for other reasons. Ranging system 1400 may also be used totrack and monitor livestock or other domesticated animals.

Reciprocal Beacon-Locator

FIG. 15 is a schematic diagram illustrating a near-field ranging systemconfigured according to a reciprocal beacon-locator architecture. InFIG. 15, a reciprocal beacon-locator ranging system 1500 includes afirst beacon-locator 1520 and a second beacon locator 1522. Firstbeacon-locator 1520 transmits a first electromagnetic signal 1515.Second beacon-locator 1522 receives first electromagnetic signal 1515and calculates a range r from first beacon-locator 1520. Secondbeacon-locator 1522 may also transmits a second electromagnetic signal1517. First beacon-locator 1520 receives second electromagnetic signal1517 and calculates range r. If first beacon-locator 1520 and secondbeacon-locator 1522 are connected via an optional data bus 1595, thenfirst beacon-locator 1520 can trigger second beacon-locator 1522 to sendsecond electromagnetic signal 1517 so that first beacon-locator 1520 candetermine range r. For purpose of illustration and not for purpose oflimitation only two beacon-locators are shown. In some applicationshowever, it may be advantageous to have additional beacon-locators sothat each member of a larger group may track or be tracked.

A variety of applications are appropriate for ranging system 1500. Forpurposes of illustration and not for limitation, a few applications arelisted below. Reciprocal beacon-locator system 1500 is useful inconjunction with two-way radios whose users desire to know how far awaya communicating party is situated. One may also advantageouslyincorporate a beacon-locator 1520, 1522 in devices that allow aplurality of people to find each other, such as parents and children atan amusement park, hunters, fishermen, or other outdoorsmen, or otherdevices in which combined tracking and communication within and amongmembers of a group is desired. Such a combined tracking andcommunicating arrangement may be useful not only for people, but alsofor vehicles, particularly aircraft and ships which may need to maintainparticular spacing or stations within a moving group. If a means fordirection finding is also used in a particular application, then bothrange and bearing information may be obtained. Reciprocal beacon-locatorsystem 1500 is also useful for allowing members of a team to monitoreach other's positions when visibility is impaired by smoke or otherintervening walls or objects. Further, reciprocal beacon-locator system1500 may be employed beneficially as part of a communication securitysystem that uses range or position information to validate orauthenticate the identity of a communicating party.

Passive Tag Architecture

FIG. 16 is a schematic diagram illustrating a near-field ranging systemconfigured employing a passive tag architecture. In FIG. 16, a passivetag ranging system 1600 includes a locator 1620 equipped with aninterrogator antenna 1638 that radiates an interrogatory electromagneticsignal 1616. In alternate embodiments, the function of interrogatorantenna 1638 may be performed by a first magnetic antenna 1631, a secondmagnetic antenna 1633, or an electric antenna 1632. Interrogatoryelectromagnetic signal 1616 is detected by an interrogatory antenna 1639of a passive tag 1629. Passive tag 1629 collects energy frominterrogatory electromagnetic signal 1616 and re-radiates the collectedenergy as an electromagnetic signal 1617 via a passive tag transmitantenna 1635.

Interrogatory electromagnetic signal 1216 may have a different frequencyor other different properties from re-radiated electromagnetic signal1617. Although interrogatory antenna 1639 and passive tag transmitantenna 1635 are shown as magnetic antennas they may be embodied inelectric antennas. Further, passive tag 1629 may include active means tomodulate re-radiated electromagnetic signal 1617. Electromagnetic signal1617 is detected by first magnetic antenna 1631, second magnetic antenna1633, and electric antenna 1632. Locator 1620 then determines range rand possibly a bearing to passive tag 1629, using the near-fielddistance measurement teachings of the present invention.

Passive tag ranging system 1600 is a good product solution when a lowcost but high volume implementation is an important goal. Passive tag1629 may be attached to luggage, mail, assets for inventory control ortheft prevention, identification cards or other personal artifacts, or awide variety of other people or assets whose location is desired to beknown with great precision.

A variety of neighboring passive tags 1629 may be distinguished fromeach other by responsiveness to different interrogatory electromagneticsignals 1616 or by various modulations applied to respective transmittedelectromagnetic signals 1617.

Near-Field Remote Sensing Architecture

FIG. 17 is a schematic diagram illustrating a near-field ranging systemconfigured employing a near-field remote sensing architecture. In FIG.17, a near-field remote sensing ranging system 1700 includes a remotenear-field sensor 1720 is equipped with an interrogator antenna 1738that radiates an interrogatory electromagnetic signal 1716. In alternateembodiments, the function of interrogator antenna 1738 may be performedby a first magnetic antenna 1731, a second magnetic antenna 1733, or anelectric antenna 1732. Interrogatory electromagnetic signal 1716 isincident on a remotely sensed object 1719. A reflected electromagneticsignal 1717 results when an incident interrogatory electromagneticsignal 1716 reflects from remotely sensed object 1719. The properties ofreflected electromagnetic signal 1717 are dependent upon the electricaland geometric properties of remotely sensed object 1719 as well as uponrange r between near-field sensor 1720 and remotely sensed object 1719.Reflected electromagnetic signal 1717 is detected by first magneticantenna 1731, second magnetic antenna 1733, and electric antenna 1732.Near-field sensor 1720 can evaluate reflected electromagnetic signal1717 to infer properties of remotely sensed object 1719.

Near-Field Ranging Method

FIG. 18 is a flow diagram illustrating the method of the presentinvention. A method 1800 for measuring distance between a first locusand a second locus begins at a START block 1802. Method 1800 continueswith transmitting an electromagnetic signal from the first locus, asindicated by a block 1804. Method 18000 continues with receiving theelectromagnetic wave at the second locus; the second locus being withinnear-field range of the electromagnetic signal, as indicated by a block1806. Method 1800 continues with, in no particular order, (1) detectinga first characteristic of the electromagnetic signal, as indicated by ablock 1808; and (2) detecting a second characteristic of theelectromagnetic signal, as indicated by a block 1810. Method 1800continues with measuring a difference between the first characteristicand the second characteristic, as indicated by a block 1812. Method 1800continues with employing the difference measured as represented by block1812 to calculate the distance between the first locus and the secondlocus, as indicated by a block 1814. Method 1800 terminates as indicatedby an END block 1816.

Fixed beacon-mobile locator ranging system 1300 (FIG. 13), afixed/mobile locator-mobile beacon ranging system 1400 (FIG. 14),reciprocal beacon-locator ranging system 1500 (FIG. 15), passive tagranging system 1600 (FIG. 16) and near-field remote sensing rangingsystem 1700 (FIG. 17) are presented for illustration and not forlimitation. A variety of alternate configurations and combinations ofarchitectures are also possible. For example, fixed locators 1420, 1422,1424, 1426 (FIG. 14) may be embodied in a beacon-locator configuration,such as beacon-locator 1520 (FIG. 15). Fixed locators 1420, 1422, 1424,1426 (FIG. 14) may be configured to cooperatively self-survey their ownrespective positions to enable rapid deployment of a positioning,locating, or tracking system. The specific exemplary applicationsprovided in connection with each respective ranging system architecturedescribed herein should not be interpreted as precluding use of adifferent architecture for a given respective exemplary application.

In another example, passive tag 1629 (FIG. 16) may be used with anetwork of locators (e.g., fixed locators 1420, 1422, 1424, 1426; FIG.14). In addition, nothing in this disclosure should be interpreted asprecluding a ranging, positioning or locating system from usingadditional information to refine an estimate of position. Such otherinformation may include, by way of example and not by way of limitation,a history of past positions or changes of position, or information fromother sensors or sources. In particular, the present invention is wellsuited as a supplement to a GPS type tracking system. The presentinvention can extend the functionality of a GPS type tracking andpositioning system into areas where GPS signals cannot penetrate or areunavailable. Also, the present invention may be used to achieve levelsof performance not attainable using GPS alone. Nothing in thisdisclosure should be interpreted as precluding use of the presentinvention in conjunction with any other prior art techniques fortracking, positioning, or locating. Similarly, the present invention maybe supplemented by prior art systems to improve the performance of thepresent invention in areas or at ranges where the present inventionalone may not yield reliable results.

Although this disclosure has focused on a single polarization in theinterest of simplicity in explaining the present invention, it should beunderstood that the teachings of the present invention can be readilyextended to multiple polarization or polarization diverse systems withmultiple parallel receive channels, including systems employing circularpolarization. Various polarization capabilities permit the systemstaught by the present invention to accommodate a variety of orientationsbetween a beacon or passive tag and a locator.

To aid understanding the present invention, this disclosure has focusedon a narrowband continuous wave (CW) implementation of the presentinvention. It should be understood that the present invention may alsobe implemented using multiple frequencies, time domain impulsewaveforms, stepped or swept sets of appropriate frequencies, or othersignals more complicated than an individual narrowband CW signal. Forexample, a phase difference of a CW signal may be related to a timedelay, or more generally, a Hilbert transform of an arbitrary timedomain signal. Any waveform (whether a CW waveform, short pulse,impulse, or time domain waveform, chirped waveform, or other waveform)will evolve from a near-field shape to a far-field shape in a mannerthat facilitates distance measurement and positioning according to theteachings of the present invention.

Specific applications have been presented solely for purposes ofillustration to aid the reader in understanding a few of the great manycontexts in which the present invention will prove useful. It shouldalso be understood that, while the detailed drawings and specificexamples given describe preferred embodiments of the invention, they arefor the purposes of illustration only, that the apparatus and method ofthe invention are not limited to the precise details and conditionsdisclosed and that various changes may be made therein without departingfrom the spirit of the invention which is defined by the followingclaims:

1. A system for measuring distance between a first locus and a secondlocus; the system comprising: (a) a beacon device situated at said firstlocus and transmitting an electromagnetic signal; and (b) a locatordevice situated at said second locus and receiving said electromagneticsignal; said locator device being situated at a distance from saidbeacon device within near-field range of said electromagnetic signal;said locator device distinguishing at least two characteristics of saidelectromagnetic signal sensed at said second locus; said locator deviceemploying said at least two characteristics to effect said measuring. 2.A system for measuring distance between a first locus and a secondlocus; the system comprising: (a) a beacon device situated at said firstlocus and transmitting an electromagnetic signal; and (b) a locatordevice situated at said second locus and receiving said electromagneticsignal; said locator device being situated at a distance from saidbeacon device within near-field range of said electromagnetic signal;said locator device distinguishing at least two characteristics of saidelectromagnetic signal at said second locus; said locator deviceemploying said at least two characteristics to effect said measuring;said at least two characteristics being a first signal characteristicproportional to a magnetic field component of said electromagneticsignal, and a second signal characteristic proportional to an electricfield component of said electromagnetic signal.
 3. A system formeasuring distance between a first locus and a second locus as recitedin claim 2 wherein said respective locator device effects said measuringby measuring a phase difference between said magnetic field componentand said electric field component of said electromagnetic signal andemploying said phase difference to determine said distance.
 4. A systemfor measuring distance between a first locus and a second locus asrecited in claim 1 wherein said beacon device and said locator deviceare coupled in a unitary assembly.
 5. A system for measuring distancebetween a first locus and a second locus as recited in claim 2 whereinsaid beacon device and said locator device are coupled in a unitaryassembly.
 6. A system for measuring distance between a first locus and asecond locus; the system comprising: (a) a beacon device situated atsaid first locus and transmitting an electromagnetic signal; and (b) alocator device situated at said second locus and receiving saidelectromagnetic signal; said locator device being situated at a distancefrom said beacon device within near-field range of said electromagneticsignal; said locator device distinguishing at least two characteristicsof said electromagnetic signal at said second locus; said locator deviceemploying said at least two characteristics to effect said measuring;said beacon device transmitting said electromagnetic signal in responseto receiving an interrogating signal from said locator device.
 7. Asystem for measuring distance between a first locus and a second locusas recited in claim 1 wherein the system further comprises a least oneadditional locator device to include at least n locator devices in thesystem effecting said measuring to said beacon device for ascertaininglocation of said beacon device in n dimensions.
 8. A system formeasuring distance between a first locus and a second locus; the systemcomprising; (a) a beacon device situated at said first locus andtransmitting a electromagnetic signal; and (b) a locator device situatedat said second locus and receiving said electromagnetic signal; saidlocator device being situated at a distance from said beacon devicewithin near-field range of said electromagnetic signal; said locatordevice distinguishing at least two characteristics of saidelectromagnetic signal at said second locus; said locator deviceemploying said at least two characteristics to effect said measuring;said beacon device and said locator device being coupled in a unitaryassembly; said least two characteristics being a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic signal, and a second signal characteristic proportionalto an electric field component of said electromagnetic signal; saidlocator device effecting said measuring by measuring a phase differencebetween said magnetic field component and said electric field componentand employing said phase difference to determine said distance.
 9. Asystem for measuring distance between a first locus and a second locusas recited in claim 6 wherein said at least two characteristic are afirst signal characteristic proportional to a magnetic field componentof said electromagnetic signal, and a second signal characteristicproportional to an electric field component of said electromagneticsignal, and wherein said locator device effects said measuring bymeasuring a phase difference between said magnetic field component andsaid electric field component and employing said phase difference todetermine said distance.
 10. A system for measuring distance between afirst locus and a second locus; the system comprising: (a) a beacondevice situated at said first locus and transmitting an electromagneticsignal; (b) a locator device situated at said second locus and receivingsaid electromagnetic signal; said locator device being situated at adistance from said beacon device within near-field range of saidelectromagnetic signal; said locator device distinguishing at least twocharacteristics of said electromagnetic signal at said second locus;said locator device employing said at least two characteristics toeffect said measuring; and (c) at least one additional locator device toinclude at least n locator devices in the system effecting saidmeasuring to said beacon device for ascertaining location of said beacondevice in n dimensions; said at least two characteristics being a firstsignal characteristic proportional to a magnetic field component of saidelectromagnetic signal, and a second signal characteristic proportionalto an electric field component of said electromagnetic signal; eachrespective locator device of said at least n locator devices effectingsaid measuring by measuring a phase difference between said magneticfield component and said electric field component and employing saidphase difference to determine said distance.
 11. An apparatus foreffecting electromagnetic ranging; the apparatus comprising: (a) atransmitter device for transmitting an electromagnetic wave; (b) areceiver device for receiving said electromagnetic wave; said receiverdevice being situated a distance from said transmitter device withinnear-field range of said electromagnetic wave; said receiver devicecomprising a first receiver means for detecting a first characteristicof said electromagnetic wave and a second receiver means substantiallycollocated with said first receiver means for detecting a secondcharacteristic of said electromagnetic wave; (c) a measuring devicecoupled with said receiver device for measuring a difference at saidreceiver device between said first characteristic and said secondcharacteristic; and (d) a determining device coupled with said measuringdevice for employing said difference to calculate said distance.
 12. Anapparatus for effecting electromagnetic ranging comprising: (a) atransmitter device for transmitting an electromagnetic wave; (b) areceiver device for receiving said electromagnetic wave; said receiverdevice being situated a distance from said transmitter device withnear-field range of said electromagnetic wave; said receiver devicecomprising a first receiver means for detecting a first characteristicof said electromagnetic wave and a second receiver means substantiallycollocated with said first receiver means for detecting a secondcharacteristic of said electromagnetic wave; (c) a measuring devicecoupled with said receiver device for measuring a difference betweensaid first characteristic and said second characteristic; and (d) adetermining device coupled with said measuring device for employing saiddifference to calculate said distance; said first characteristic being afirst signal characteristic proportional to a magnetic field componentof said electromagnetic wave, and said second characteristic being asecond signal characteristic proportional to an electric field componentof said electromagnetic wave; said difference being a phase differencebetween said magnetic field component and said electric field component;said determining device employing said phase difference to calculatesaid distance.
 13. An apparatus for effecting electromagnetic ranging;the apparatus comprising; (a) a transmitter device for transmitting anelectromagnetic wave; (b) a receiver device for receiving saidelectromagnetic wave; said receiver device being situated a distancefrom said transmitter device within near-field range of saidelectromagnetic wave; said receiver device comprising a first receivermeans for detecting a first characteristic of said electromagnetic waveand a second receiver means substantially collocated with said firstreceiver means for detecting a second characteristic of saidelectromagnetic wave; (c) a measuring device coupled with said receiverdevice for measuring a difference between said first characteristic andsaid second characteristic; and (d) a determining device coupled withsaid measuring device for employing said difference to calculate saiddistance; said transmitter device transmitting said electromagnetic wavein response to receiving an interrogating signal from said receiverdevice.
 14. An apparatus for effecting electromagnetic ranging asrecited in claim 13 wherein said first characteristic is a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic wave, and said second characteristic is a second signalcharacteristic proportional to an electric field component of saidelectromagnetic wave, and wherein said difference is a phase differencebetween said magnetic field component and said electric field component;said determining device employing said phase difference to calculatesaid distance.
 15. A method for measuring distance between a first locusand a second locus; the method comprising the steps of: (a) transmittingan electromagnetic signal from said first locus; (b) receiving saidelectromagnetic wave at said second locus; said second locus beingwithin near-field range of said electromagnetic signal; (c) in noparticular order: (1) detecting a first characteristic of saidelectromagnetic signal at said second lotus; and (2) detecting a secondcharacteristic of said electromagnetic signal at said second lotus; (d)measuring a difference at said second locus between said firstcharacteristic and said second characteristic; and (e) employing saiddifference to calculate said distance.
 16. A method for measuringdistance between a first locus and a second locus; the method comprisingthe steps of: (a) transmitting an electromagnetic signal from said firstlocus; (b) receiving said electromagnetic wave at said second locus;said second locus being within near-field range of said electromagneticsignal; (c) in no particular order; (1) detecting a first characteristicof said electromagnetic signal at said second locus; and (2) detecting asecond characteristic of said electromagnetic signal at said secondlocus; (d) measuring a difference between said first characteristic andsaid second characteristic; and (e) employing said difference tocalculate said distance; said first characteristic being a first signalcharacteristic proportional to a magnetic field component of saidelectromagnetic signal; said second characteristic being a second signalcharacteristic proportional to an electric field component of saidelectromagnetic signal; said difference being a phase difference betweensaid magnetic field component and said electric field component.
 17. Amethod for measuring distance between a first locus and a second locus;the method comprising the steps of: (a) transmitting an electromagneticsignal from said first locus; (b) receiving said electromagnetic wave atsaid second locus; said second locus being within near-field range ofsaid electromagnetic signal; (c) in no particular order: (1) detecting afirst characteristic of said electromagnetic signal at said secondlocus; and (2) detecting a second characteristic of said electromagnetsignal at said second locus; (d) measuring a difference between saidfirst characteristic and said second characteristic; and (e) employingsaid difference to calculate said distance; said transmitting saidelectromagnetic signal occurring in response to receiving aninterrogating signal.
 18. A method for measuring distance between afirst locus and a second locus as recited in claim 17 wherein said firstcharacteristic is a first signal characteristic proportional to amagnetic field component of said electromagnetic signal, and said secondcharacteristic is a second signal characteristic proportional to anelectric field component of said electromagnetic signal, and whereinsaid difference is a phase difference between said magnetic fieldcomponent and said electric field component.